OK, select the cells you want to replace the uppercase values with lowercase values and run the macro below.

Sub Sample()
    Dim rng As Range
    Dim str As String
    Dim Matches
    Dim Match
     
    For Each rng In Selection
        str = rng.Value
        With CreateObject("VBScript.RegExp")
            .Pattern = "\d.+\d"
            .Global = True
            Set Matches = .Execute(str)
            If Matches.Count > 0 Then
                For Each Match In Matches
                    str = Replace(str, Match.Value, LCase(Match.Value))
                Next Match
            End If
            rng.Value = str
        End With
    Next rng
    
    Set Matches = Nothing
End Sub