Wow-lots of changes for that little toddler! Try not to rush it too much, at least do it in stages/"baby steps" as to no over-whelm her! If she feels that she is being pushed out due to the baby, resentment to either you two or the baby will arise.
My kids are 17 mos. appart. (That made my oldest, now almost 3 yrs old, almost 17.5 mos. old at the birth of my son, now 1)
We ran into this issue, though not the separate floor issue. What we did may help you out quite a bit!
This is what we did!
First, we moved the CRIB into the new room (Big kid room) as well as ANYTHING and EVERYTHING comforting to her and familiar (toys, wall hangings, other furniture, etc)and put her in there in the crib for NAPS only! The familiar objects are craved by toddlers. She STILL slept in the nursery for bed/night (in the port-a-crib/pack-n-play: if you don't have one, borrow one for a just a temporary time-frame) for about 2 weeks and the baby slept still with us in the basinet in our room.
Then, when she was used to napping in her new room, we had her do all sleeping in her new room, BUT STILL IN THE CRIB (this is what she is used to, the crib): a new room AND a new bed is scary for a young child! We also from time-to-time ended up with one parent on the bed in the new room for a couple of hours in the night (either at bed time or when the toddler woke and needed comforting) so she did not feel alone (all by herself upstairs in your case). You can then move the baby in the nursery (if you wish) in the pack-n-play! Newborns don't need a crib with a mattress anyways as they usually find a corner and "cuddle" in it. That is why the close-ness of the basinet is comforting to them. Maybe you could even put the basket-part of the basinet inside the pack-n-play for naps at first if the newborn seems un-happy.
Then, we finally moved the todler into the new bed but still left the crib in there for "sight support" for a bit! When she seemed comfortable, just one day, the crib dissappeared (in our closet) and was gone for about a week.
Then, all of the sudden one day, the crib re-appeared as different looking as possible in the nursery (different bedding/colored sheets, etc) and all was well.
Baby steps always seemed to work very well with all our transitioning! She did AWESOME in a todler bed!!!! She never had a problem falling asleep and we never had a problem keeping her in it. She is now in a big kid bed (as of last week) and doing just great (we left her todler bed in there for the time being)!
Good Luck!
As far as stair safety (we are all upstairs in our house), we have a WALK-THROUGH baby gate at the top of the stairs (we fashioned a door frame and hooked it to that. It is clear and opens like a small door (baby-gate height). And we have a reg. baby gate at the bottom. I have a monitor in both of the kids rooms (duel base, duel receiver) with the receiver down stairs in the kitchen (and I can take it with me wherever I go) so I can hear them during nap upstairs.