B.B.
We are planning on doing this soon as well since we're trying for #2. My son is also 18 months. I've read on other posts on here that you should definitely have a baby gate on his door, so he can't just wander around the house if he gets up in the middle of the night. You're his mom, so you know if he's ready for a toddler bed. All kids are different, so it's really up to you. Good luck!
I just remembered a cute story my mom told me about the first night they moved my older brother to a big boy bed. They found him the next morning in the middle of all his stuffed animals in the corner, kinda like in "ET". She said it was the cutest thing! He felt safer there, I guess!