We literally just went through this. The initial plan was to move DS (26mos) to his big boy bedroom. We had a twin bed, new decorations, all of it. And after months of trying (we started trying to move him at 18mos), he just wasn't ready.
So, we finally decided to keep DS in his crib and just buy used nursery furniture for our newborn.
Then, DS started crawling out of his crib. Like, a month later. Constantly. So in the end, we wound up converting his crib to a toddler bed. It's been 2 weeks now, and I think we're through the rough part (a few VERY long and unpleasant nights of DS not staying in bed, not wanting to be alone, etc...). The newborn is coming on Friday (induction), so we'll see how that goes!
In the end, I think, keep the kids in the crib as LONG as you can. I would've kept DS in his until he was 5! He just didn't give us a choice, in the end.