I think any Y fare (including Y2s) are eligible. The AA site specifies Y, Y2 and YX2. I'd imagine the business class fares have to be J or J2. It;d be nice if one could upgrade from a C or D fare.

If the upgrades are R/T, they are quite good value (actually, tedhl is correct as it's comparable to AA's full-fare upgrades for comprable destinations). I'm just used to looking for upgrades on cheap tickets.