Moderator, Marriott Bonvoy & FlyerTalk Evangelist
Join Date: Oct 2002
Location: McKinney, TX, USA
Programs: United Silver; AA Plat/2MM; Marriott LT Titanium; Hilton Gold
Posts: 11,727
The "upgrade" cost is the difference between what the TP Cat. level you purchased versus what you want to upgrade to. (So no savings or loss, just pay the difference.)
I have never tried to upgrade a hotel cert from Cat to Tiers, but I don't see why they wouldn't do it (while charging you the difference it cost of course.) It might not hurt to call MR to verify that they will do it though.