OP - You could get really lucky like a friend of mine did, and your child won't realize he can get out of the bed. If that happens, keep it a smug secret like you would if your child started STTN at 2 weeks
We do the lead child silently back to the bed routine. The first week it was almost 10 times a night. Now he stays in bed until he hears one of us in the shower, or at most wakes up once during the night - most often because he's kicked all his covers off and he's cold. I wouldn't be comfortable gating him in his room, but we do have a gate at the top of the stairs, so he can wander into our room or the bathroom, but not all over the house.
Also, I would do the transition as soon as possible so he doesn't immediately link it with the new baby. We're doing that as we move our son from his toddler bed to a twin bed, and talking in terms of getting his own big boy room, rather than needing to turn his bed back into a crib.