I had a similar issue with multiple schedule changes by LH on a *A F award (SYD-SIN-MUC-ZRH-MUC-SIN-SYD) - each time UA called me to try and work new itin - while using SQ (A380 or 77W) flights would have made the new schedule / itin work SQ would only provide Y on these planes. After more than an hour on the phone ended up with SYD-SIN-MUC-stopover-ZRH-FRA-SIN-stopover-SYD all in F (or C in SQ) and one flight on 77W. Note UA allowed an additional stopover so I could get F or C on SQ. Agents, as there were 2 changes, were most helpful and indicated that there are many many schedule changes by various *A carriers causing lots of issues for them.