Michelle-
We just did the exact same thing with Tyler, who's 2 mo older than Ben. We moved rooms and switched him to his new bed all at one time on Saturday. We left the old crib up in his old room, but the door is closed and we NEVER open it, and he doesn't know how to open doors yet. If he did, I would have taken it apart and put it away so he couldn't see it anymore. We also called his bed his "new crib" and that seemed to help him understand that this is where we sleep now. For now, we're letting him have whatever toys he wants in there. When he comes out, we pretty much do what Florencia said. We simply tell him it's time for night night, so let's go back in the crib and we take him back up to bed.
In all honesty, it may just take time. I really wouldn't go back to the crib, even if he's upset. With only 8 wks to go, you could let the baby sleep just about anywhere at first, but I don't think moving Ben will get any easier with a new baby getting ready to sleep in HIS crib. It's tough, but you have to move him sometime and with your help, he can feel just as secure and happy in his new bed. Keep us posted!