Buy a bigger bed :=)

Now it's a bit different for us as DS was 4yo when DD was born and didn't co-sleep as much anymore, it was more a 3-4am crawl in snuggle. Having said that there have been plenty of times come midnight we have had both in our bed, I usually sleep between them althought after DD was 1+ they did like sleeping next to each other.

We would put DS in to his own bed and then bring him in to cosleep if he woke so we didn't have the concern with new beds and stuff. If your eldest is used to cosleeping then I would leave them there, at least until a while after the baby was born. I would have a second bed mattress in case anyone you, bub or DS needs some space and tell DS that he has his very own bed, if he wants to sleep there - he might like the idea of having his own space away from the baby.

DS and DD share a room here and love it, the noise has never been a problem for DS? and it's nice now to be able to put them both to bed and be there for both of them and of course they have each other as well