Here are a few VBA functions that I cooked up to replace all instances of a selected portion of string within string. I am sure that this isn't anything new but it's my take on it and it may save some people some time from having to figure it all out.

Aside from calling this in VBA Code, you can also use the VBA function in a Microsoft Access query or control on a form. The sample file demonstrates how it can be used in a spreadsheet.

ReplaceInStr() [Same function as Replace(), but I wrote this as an exercise]
Example: Replace all instances of "X" with "1"
i.e. =ReplaceInStr("X+1","X",1) = "1+1"

MultiReplaceInStr()
Example: Replace all instances of "A" with "1", "B" with "2", and "C" with "3". Must create two strings; one with the 'to-be-replaced' strings as a single string with commas separating the individual strings, and similarly one with the 'replacement strings'. The first replacement is done for the entire string first, then the second once the first is done, and so on.
i.e. =MultiReplaceInStr("A+B+C","A,B,C","1,2,3") = "1+2+3"

MultiReplaceInStrArray()
Example: Replace all instances of "A" with "1", "B" with "2", and "C" with "3". First provide a 'to-be-replaced' string, then the 'replacement string, and so on. The first replacement is done for the entire string first, then the second once the first is done, and so on.
i.e. =MultiReplaceInStrArray("A+B+C+ABC","A","now I know my ABC","A",1,"B",2,"C",3) = "now I know my 123+2+3+now I know my 12323"

CountInStr() - Used by MultiReplaceInStr()
e.g. CountInStr("q,w,e,r,t,y",",") = 5

VBA Code

Option Compare Database 'Only needed for Microsoft Access VBA
Option Explicit


Public Function ReplaceInStr(ByVal strString As String, _
                             ByVal strReplacee As String, _
                             ByVal varReplacer As Variant) As Variant

    '**************************************************************
    'ReplaceInStr()
    'Written by Azli Hassan
    '
    'PURPOSE:   To replace all instances of a given string
    '           within a string.
    '
    'ARGUMENTS:
    '1) strString (String)
    '   - String te be changed
    '2) strReplacee (String)
    '   - String to be replaced
    '3) varReplacer (Variant)
    '   - Variant to replace strReplacee in strString
    '
    'RETURNS:   (Variant) - Either a string or an error value
    '   e.g. ReplaceInStr("X+1","X",1) = "1+1"
    '**************************************************************

    On Error GoTo ErrMsg:

    Dim lngPos As Long, lngRight As Long, _
        strLeft As String, strRight As String, _
        strReplaced As String
    strReplaced = strString
    lngPos = InStr(1, strReplaced, strReplacee)
    While lngPos <> 0
        strLeft = Left(strReplaced, lngPos - 1)
        lngRight = Len(strReplaced) - lngPos - Len(strReplacee) + 1
        strRight = Right(strReplaced, lngRight)
        strReplaced = strLeft & varReplacer & strRight
        lngPos = InStr(lngPos + Len(varReplacer), strReplaced, strReplacee)
    Wend
    ReplaceInStr = CStr(strReplaced)

ExitHere:
    On Error Resume Next
    Debug.Print "ReplaceInStr = " & ReplaceInStr
    Exit Function

ErrMsg:
    Debug.Print Err.Number & " - " & Err.Description
    ReplaceInStr = CVErr(Err.Number)
    Resume ExitHere:
End Function


Public Function MultiReplaceInStr(ByVal strString As String, _
                                   ByVal strMultiReplacee As String, _
                                   ByVal strMultiReplacer As String, _
                                   Optional strDelimiter As String = ",") _
                                   As Variant

    '**************************************************************
    'MultiReplaceInStr()
    'Written by Azli Hassan
    '
    'PURPOSE:   To replace all instances of a
    '           multiple given strings within a string.
    '
    'ARGUMENTS:
    '1) strString (String)
    '   - String te be changed
    '2) strMultiReplace (Array of type Variant)
    '   - Delimited string of string parts to be replaced
    '3) strMultiReplacer (String)
    '   - Delimited string of replacement strings
    '3) strDelimited (String)
    '   - Delimiter string (Default = ",")
    '
    'Example:
    '   MultiReplaceInStr("A+B+C","A,B,C","1,2,3") = "1+2+3"
    '
    'RETURNS:   Either a string or an error value
    '**************************************************************

    On Error GoTo ErrMsg:

    Dim lngCountReplacee As Long, lngCountReplacer As Long
    'Check that the number of string parts to be replaced
    'is the same number of replacement strings
    lngCountReplacee = CountInStr(strMultiReplacee, strDelimiter)
    lngCountReplacer = CountInStr(strMultiReplacer, strDelimiter)
    If lngCountReplacee <> lngCountReplacer Then
        Err.Raise 13
    End If

    Dim iReplacee As Long, iReplacer As Long
    iReplacee = 1
    iReplacer = 1

    Dim i As Long
    Dim lngPosReplacee As String, lngPosReplacer As String
    Dim strReplacee As String, strReplacer As String
    Dim lngMultiReplacee As String, lngMultiReplacer As String

    MultiReplaceInStr = strString

    For i = 1 To lngCountReplacee + 1
        'Find Position of delimiter
        lngPosReplacee = InStr(iReplacee, strMultiReplacee, strDelimiter)
        lngPosReplacer = InStr(iReplacer, strMultiReplacer, strDelimiter)
        'Get replacee string
        If lngPosReplacee = 0 Then
            strReplacee = strMultiReplacee
        Else
            strReplacee = Mid(strMultiReplacee, iReplacee, lngPosReplacee - iReplacee)
            lngMultiReplacee = Len(strMultiReplacee) - lngPosReplacee
            strMultiReplacee = Right(strMultiReplacee, lngMultiReplacee)
        End If
        'Get replacer string
        If lngPosReplacee = 0 Then
            strReplacer = strMultiReplacer
        Else
            strReplacer = Mid(strMultiReplacer, iReplacer, lngPosReplacer - iReplacer)
            lngMultiReplacer = Len(strMultiReplacer) - lngPosReplacer
            strMultiReplacer = Right(strMultiReplacer, lngMultiReplacer)
        End If
        'Replace
        MultiReplaceInStr = ReplaceInStr(MultiReplaceInStr, strReplacee, strReplacer)
    Next i

ExitHere:
    On Error Resume Next
    Debug.Print "MultiReplaceInStr = " & MultiReplaceInStr
    Exit Function

ErrMsg:
    Debug.Print Err.Number & " - " & Err.Description
    MultiReplaceInStr = CVErr(Err.Number)
    Resume ExitHere:
End Function


Public Function CountInStr(ByVal strString As String, _
                           strFind As String) As Long
    '**************************************************************
    'CountInStr()
    'Written by Azli Hassan
    '
    'PURPOSE:   To count all instances of a
    '           given string within a string.
    '
    'ARGUMENTS:
    '1) strString (String)
    '   - String te be changed
    '2) strFind (String)
    '   - String to find and count within strString
    '
    'Example:
    '   CountInStr("q,w,e,r,t,y",",") = 5
    '
    'RETURNS:   Either a string or an error value
    '**************************************************************
    On Error GoTo ErrMsg:

    Dim lngPos As Long, lngRight As Long, _
        strLeft As String, strRight As String
    CountInStr = 0
    lngPos = InStr(1, strString, strFind)
    While lngPos <> 0
        CountInStr = CountInStr + 1
        strLeft = Left(strString, lngPos - 1)
        lngRight = Len(strString) - lngPos - Len(strFind) + 1
        strRight = Right(strString, lngRight)
        strString = strLeft & "" & strRight
        lngPos = InStr(1, strString, strFind)
    Wend

ExitHere:
    On Error Resume Next
    Debug.Print "CountInStr = " & CountInStr
    Exit Function

ErrMsg:
    Debug.Print Err.Number & " - " & Err.Description
    CountInStr = CVErr(Err.Number)
    Resume ExitHere:
End Function


Public Function MultiReplaceInStrArray(ByVal strString As String, _
                                       ParamArray aReplace() As Variant) _
                                       As Variant
    '**************************************************************
    'MultiReplaceInStrArray()
    'Written by Azli Hassan
    '
    'PURPOSE:   To replace all instances of a
    '           multiple given strings within a string.
    '
    'ARGUMENTS:
    '1) strString (String)
    '   - String te be changed
    '2) aReplace (Array of type Variant)
    '   - Array of string to be replaced and replacement string.
    '     Order of array is
    '       i)  String to be replaced
    '       ii) Replacement string
    '
    'Example:
    '   MultiReplaceInStrArray("A+B+C","A",1,"B",2,"C",3) = "1+2+3"
    '
    'RETURNS:   Either a string or an error value
    '**************************************************************

    On Error GoTo ErrMsg:

    If (UBound(aReplace) - LBound(aReplace) + 1) Mod 2 <> 0 Then
        Err.Raise 13
    End If

    Dim i As Long
    MultiReplaceInStrArray = strString
    For i = LBound(aReplace) To UBound(aReplace) Step 2
        MultiReplaceInStrArray = MultiReplaceInStr(MultiReplaceInStrArray, CStr(aReplace(i)), aReplace(i + 1))
    Next i

ExitHere:
    On Error Resume Next
    Debug.Print "MultiReplaceInStrArray = " & MultiReplaceInStrArray
    Exit Function

ErrMsg:
    Debug.Print Err.Number & " - " & Err.Description
    MultiReplaceInStrArray = CVErr(Err.Number)
    Resume ExitHere:
End Function

Sample File

Sample file (Microsoft Excel) can be downloaded from here.