If you have a TA-issued ticket I'm assuming all changes prior to travel need to be arranged with the TA. Once travel has commenced can you just go to any OW airline or does it have to be the one the whole ticket is plated to?

... And... Let's say certain segments are using the code share, rather than the OP carrier, is there any way to make sure that sticks? I've read that BA, QF and AA are sticklers for making you use their prime flight number where that's relevant. I'd be looking at date, rather than routing changes.