Transition to toddler bed?

My DS will be 18 months when this baby arrives (if it's not a month early like its brother) any mommas with littles that close? Did you transition to a toddler bed or purchase another crib? I'm really torn. I don't think we can fit two cribs in the room we will have them in until we can buy a new house. But I also don't want to move him too early and ruin his sleeping habits. 😕