What do you mean with the "uppercase legacy"? You mean all the current documents we already have?
In my case, those will remain with the upper case tags until I need to edit them.
I guess it would be enough to patch the sites affected by
https://code.orgmode.org/bzg/org-mode/commit/13424336a6f30c50952d291e7a82906c1210daf0
and also the templates definition in org-tempo.
Looking at that diff, the changes are not so simple as I initially thought, I was just thinking about patching org-tempo, but I guess they should cover all the uppercase tags.
Now I don't feel so confident about doing it :-(
Regards