I was going to mention this a little while back, but thought I'd wait until this episode when it's explicitly stated. Barnabas clearly tells Quentin that in order for Jeb to change to his true form, he has to be in the special room.
Presumably that would apply to his younger selves. Yet, when Alexander terrorized Paul, first with the breathing and then becoming the young Carolyn, and when Michael chased Maggie into the secret passages and later when he returned, we clearly heard the breathing and presumably he had changed.
We saw them both at Collinwood, as Alexander and as Michael, before these incidents, so he clearly changed form there.