First, do not transition your older child to a bed very near the time of the new baby. You either need to do it now and let it get well established, or wait (as another person suggested) until after and use a bassinet in the mean time.
I transitioned my daughter to her toddler bed (she has the convertable crib) at 16 months- it was probably too early, but she did well for the first two weeks. She goes to bed really well- we have a really well established bedtime routine and she puts up absolutely no fight at bedtime, and she goes to sleep completely on her own. I bought a rail for the side of the bed so she wouldn't fall out, and it left just enough room for her to crawl in and out of the bed. She LOVED her big girl bed and we had no problems for two weeks. Then she started getting up over and over and over- I don't put up with much, each time I put her right back to bed with no communication or reward for getting up, but eventually she would get up and fall asleep on the floor behind her door- moral of the story- the hype we used worked even for her, but she was too young so now shes back in the crib. Make sure you set very strict limits and back them up. Put him in bed once according to the normal routine, then if he gets up guide him back to bed without any comforting or conversation. You may have to do this several times. Then if he gets up in the night (and he eventually will) guide him right back to bed again like before- keep in mind that this is often where parents cave and let the kid come to bed with them.
Good luck- just be firm but positive and don't put too much pressure on the little guy. Having a new baby sibling can be pretty hard on a toddler!