As I recall it: U class. Anyway, you will not receive Avios or TP for the upgraded RJ flights - you need to request it from BAEC afterwards (and based on original booking class). This issue had been addressed a few times in the BA forum. Moreover, most likely only the ex AMM flights will post automatically, so you also need to contact BAEC to get Avios and TP for flights to AMM. This is also covered in the BA forum, so I suggest to try searching there.
The above is based on my RJ experience this summer where I successfully upgraded the AMM-BKK leg (but so far unsuccessful in receiving Avios and TP for this flight and for the 2 other RJ flights to AMM - BAEC is looking into it now). FYI I paid USD 340 for the upgrade to BKK (which was minimum bid for my flight) so no need to bid more than the minimum amount.