I am looking at purchasing a M outbound, B inbound TATL ticket because both allow for upgrades for only 25k miles each. I don't need the flexibility afforded by a B on the outbound, so I was trying to lower the costs a bit (at the sacrifice of MQM).

I know that you should be limited by the more restrictive of the two. Is this the case even after I fly the outbound? I'd rather avoid a YMMV/HUACA situation should I wish to change the return.