For a while you have been able to switch puzzles in the middle of a trade, now it is a bit better.
We had been keeping track of missing pieces during the switch process, but puzzle notes were not displaying. Now that is fixed and both the missing pieces (if any) and the puzzle note are logged into the message thread for historical purposes.
This is important for those cases where you start a trade, then realize the puzzle you picked is not really the one you want and you need to switch. Of course this is all before shipping happens. But we wanted to make sure that all the important information made it on screen and into the message thread so no one gets unexpected surprises along the way.