As alc mentions, upgrading a TP hotel cert from a lower Cat level to a higher one shouldn't be any issue at all. Lots of posters have done it (my current TP hotel cert sitting in my account right now was originally a Cat. 5, but I upgraded it to a Cat. 8.)

But I don't remember seeing any posts (though there may well be some) of someone upgrading from a Cat level to a Tier level hotel cert. My guess is it shouldn't be a problem, but not completely sure. I'd suggest calling MR and asking if they would do that. While one agent saying they will do something doesn't mean that all of them would, it would be a fairly good indicator that there shouldn't be an issue.

Hopefully, someone whose attempted it will chime in and let us know.