Now I can see why you don't want to stir up trouble!
I think you should visit the school and see what the 3 year old room looks like (and the 4 year old room while you are at it) and do ask the director to sit in on class. That doesn't seem like alot.
As for a midyear move - I think alot depends on the personality of the kid. If you kid makes 'friends' easily at the playground then I can't imagine it would be a problem. Afterall, at this age what passes for 'friendship' is hardly what we adults think of as friendship. But I come from an unusually extroverted family, so perhaps I'm not the best to get advice from. If you want to 'stack the odds' then get a kid from the room your son will be entering and invite him over for a playdate on a weekend. Repeat 3 more times with different kids if you are nervous. That might be all it takes so your son doesn't have to walk into a room totally 'fresh.'
Best Wishes,
Grinity
