Thanks for all the responses.^

My worry about losing my seat assignments were simply a response to the message that pops up when you try to cancel your check-in (something about losing your seat assignments and processed upgrades, etc...). I did successfully cancel OLCI and checked in again, with no upgrade offered.

I'll ask at Suvarnabhumi in the morning, but I'm not too hopeful. If it happens, it happens, if not, I'll be happy with my current seats.