While I didn't have to move my son upstairs, I did move him for the baby. We painted the room (it was one that normally had the door closed), and occasionally invited him in. He LOVED to play in it. I asked him if he would like to have THAT room? He got excited for it and built him up to it. When he came home from daycare one day, we were able to show him his new room. (My husband had moved his crib during the day.) Now that he had this new, cool, big boy room, did he want to give his old room to the baby? He did. We didn't put the baby immediately in that room, so that he wouldn't feel pushed out, but as soon as she was born, we started calling it her room, and would change and dress her in there. It has been really smooth.
Do you have a monitor for him? You can use the gate to keep him in his room, and have a monitor. Let him know that this is a special big boy room, and that he needs to stay in it (or in the bed - that's what we do) until you come get him, but that you can hear him through the monitor if he needs you. And DEFINITELY get a gate for the stairs. When we moved my son to a toddler bed, we let him know that if he got out of bed before we got him up, he would have to go back to a crib.