I can’t give you a specific CL# as the change was made in one of our internal game branches so came back in a merge.
If you have 4.16 source code you can take a look for FTextHistory_Transform and apply that class and its usage to 4.15. Thankfully Transform was the next entry in ETextHistoryType (after AsDateTime) so you shouldn’t run into any serialisation issues there. Also check out UKismetTextLibrary for the Blueprint exposure of FText::ToUpper and FText::ToLower.