Consistency, consistency, consistency!!!!
I'm having the same problems as we've just transitioned to a BBB and at nap time DS (2yr 9 mo) gets up and comes out of his room many, many times (as we speak*sigh*!)
Strangely enough he goes straight to bed at night for DH
So, I just return him to his bed without any communication or eye contact and repeat until he goes to sleep!!!
I have successfully done WI/WO with him too recently to get back on track after illness etc. It usually takes about a week.
I would say it's better to sort it out now if you have the energy before the new babe comes along!
HTH
Lis