Was he having problems in the crib? Was he climbing out? Or...is the only reason you're wanting to change is because you need the crib for the baby?
If he's doing fine in the crib, leave things the way they are. Let's see: you just brought home another child, your first child used to be the absolute center of your universe and now he has to share the spotlight, you mention he has been regressing a bit...and now you want to take away his bed and give it to the baby - his perceived 'source' of all this change? That's a lot of change and stress for a 2 1/2 year old!
If you are going to insist on this move (you're a brave woman; I can't imagine doing this with a newborn and being all sleep-deprived!) then you will definitely want to employ a very easy (but potentially very trying) technique.
Switch to the bed. Tell him he is to stay in bed all night. Go through your normal routine and then put him to bed. Put a chair outside the door (this is for you to sit in when he gets out of bed fifty million times). When he gets out of bed, calmly and without talking or making eye contact, put him back into bed. No talking, no emotion, no justification, no begging, no pleading, no punishing - these only cause your child to get more and more wound up and awake and more likely to continue the pattern of behavior. He might get up over a hundred times the first night. Take shifts with your husband and be painstakingly consistent - your son needs to know that getting out of bed is not worth the effort (nothing fun is going on and it is all boring) and that every time he gets out of bed, he will be sent back to bed (thus he learns that the behavior isn't worth his effort because he's only going to go back). You can try a baby gate, but we tried it once and our son jumped over it with ease. Other people have used protective door covers so the kid couldn't get out of the room; we didn't try that and hoped to achieve success without trapping him in the room.
This may take some time - for us it took about a week and a half, and we also learned that he stayed in bed better for daddy than he did for me. We also did a sticker/reward system. For every night he stayed in bed he got a sticker. Once he got to five stickers we put Thomas sheets on his bed (he was so excited). Now after about a month of this, the novelty of the sticker has worn off but not the responsibility of staying in bed!