Since the last patch you can no longer edit your squads from the lobby, but you can still swap between already-defined squads. However this doesn’t seem to work all the time. On three occasions last night I changed my squad in-between maps but still ended up with the same one as in the previous game. After the first instance I was careful to check that the lobby screen correctly reflected my swap after doing it (correct merc portraits in the squad box and in the team list), but things always looked OK there, even those times when I ended up with the wrong squad in-game 
So far I don’t see a pattern as to when this happens vs swapping as expected.