One way to do this is via a very short piece of VBA code. Copy & paste the following into a code module, then assign it to a new custom toolbar button.
Private Sub UpperCase()
Dim cell
If TypeName(Selection) <> "Range" Then Exit Sub
For Each cell In Selection
If cell.HasFormula = False Then cell.Value = UCase(cell.Value)
Next cell
End Sub
Then, when you want to convert a range to upper case, simply select the range and click the cutom toolbar button. You can of course replace the 'UCase' function with 'LCase' to convert to lower case or 'Worksheetfuntion.Proper' to convert to proper case.