As far as sleep goes, if it's not broke, don't fix it, has always been my parenting mantra. So many things upset sleep (developmental leaps, nap transitions, A time fluctuations, sickness, teething....) I couldn't bare to mess with it if at the moment, it was working.
What do you mean "struggling with the small space?" Is he getting caught in between bars, hitting his head? When I think about the transition to a BBB, I consider emotional maturity more than anything. I feel they need to have enough impulse control to hopefully stay in bed, and the ability to reason with me on why staying in bed is important/safest. We just moved DS2, and he is 34 months. I think it's a totally individual choice. For us, at 25 months there was no way DS would have been able to handle a BBB.