We didn't. DD1 asked for undies at night when she started wearing them during the day so we just went with it because well, we just thought let's she what happens rather than try and fight with her about it every night. We figured she would either be fine or would wet and decide to stay in pull ups a bit longer for comfort so it would sort itself out. She actually never had a problem, gets up in the night to use the toilet occasionally and very rarely wet the bed... maybe 5 times total and she's almost 5 now. So guess we got lucky.

"Toilet training" was my biggest parenting fear