Thanks ChrisA330 and mendy7511 for the replies. I guess I confused things with the routing - I was trying to emphasize whether a particular booking with different fare classes could be booked. Let me retry it.

If the route was

YVR (AC) --> LHR
LHR (LH) --> FRA
FRA (LH) --> JNB

could a travel book the AC leg in B (so I could instant upgrade if available) and the LH segments in U (lowest economy) or does the fare class have to be the same for all flights under a PNR?

Thanks