DS1 was 2 and the transition went very smoothly. Like some others have said, he has always called for us instead of getting out of bed. Even now at 3.5 he will call for me in the middle of the night (but does get up on his own in the mornings which he started doing around 3). I am not sure what to do about DS2 TBH. He will be 26mos when the baby is born, and while we do need the crib for the baby I bought a cradle that would probably buy me almost 6 more months, so DS2 can stay in the crib potentially until 2.5+yrs. I *think* he would do the transition smoothly as he is an angel sleeper and loves to sleep, but at the same time I feel like he does so well in his crib so why change it, kwim? Also, we plan on moving the boys into the same room soon and I don't think we should do a BBB transition at the same time, rather we could move his crib into the room with DS1 and he can sleep there until they are used to being together. I don't know though, so many decisions!! And now I'm just talking my own issues out loud, lol.
Good luck with what you decide!!