I did kind of the opposite thing. We have a HUGE basement/living room area (picture a large L shape) that I was using for daycare when both my daughter and my daycare boy were infants. The big part of the area was their play area with all their toys and my desk. We just had one couch and TV in the smaller part of the area. They both napped in my daughter's room down the hall.
Once they started crawling, I found it incredibly hard to come up with a proper gating solution to keep them where I wanted them to play without having to add tons of gates to the downstairs...it's an awkward shape. Also, I had to carry 2 babies upstairs every time I was going to feed them and back downstairs and upstairs for naps twice a day. Not impossible, but definitely annoying.
We have a nice large room right off the kitchen upstairs that was our "den/tv room". We since moved all of the furniture out of that to the downstairs and now have a big rec room/tv room downstairs. The old den is now the playroom with an easy gate at the doorway. It has been so much better to have them right off the kitchen. I turned one of the upstairs bedrooms (my step son's) into nap room during the week. Now, I never have to go downstairs during daycare times and at the end of the day, that is where we relax. It's great because I don't feel like I'm still at work at the end of the night. My daughter has her some of her toys downstairs that she doesn't have access to during the day, so it's special for her at the end of the day, too.
With that said, now that my kids are both walking, I could do daycare downstairs pretty easily. But I'm not changing it around again!
The only thing I would consider heavily is how much it means to you, currently, having the kitchen on the same level as the kids. That has been the biggest benefit to me in switching my areas around is having the kitchen on the same level as where the kids. Kids are eating? Great, I can do a load of dishes. One kid is done eating and the other isn't? Doesn't matter because I can let the other one finish eating and the other one can go play. I couldn't do this if I still had my playroom downstairs and we had to go upstairs for meals.