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