We went through this last year. When my son was 16 months old, we took the front of his crib off to make a toddler bed. The first few nights were rough since he now had all this freedom. We did a lot of putting him back in bed in the beginning (I let my husband do most of it since he was less likely to cave than I was :)), but we stuck to it. After a week, he was staying in bed and sleeping just as good as ever (he's always been a good sleeper).
When he was 18 months old we moved him to a different room in a twin bed. He had no problems with this transition. The baby was born when my oldest was 19.5 months old, and he had no problems/relapses with staying in bed. He had also "forgotten" that the baby now had his bed so we didn't deal with jealousy.
I totally suggest going ahead and making the transition now. 2-3 months is enough time to get him trained in a big boy bed. You'll have a lot more time and patience now than when the baby comes. If you put if off until after the baby is born, when you do decide to make the transition, you'll have to deal with the crying and getting out of bed while you have a baby around that he can disturb.
Good luck!