I would keep it same cal unless of course chamber is pitted,
from another site Quoting here:
After looking at cartridge dimensions... I'm pretty sure even the standard .220 Swift reamer could be run into the .225 chamber without an issue. If so; a good gunsmith should be able to rechamber the rifle without even setting the barrel back.
If I have over looked something... I guarantee .220 Swift Ackley Improved would nullify the .225 for a rechamber. You would also have a much easier time reloading for the rifle. (The power/velocity gain isn't worth it, just the brass-flow factor.) Trimming Swift brass is a bit irritating, and never- ending. Neck reaming/turning is the real issue for most reloaders, though. I can't get more than 3 firings out of my brass before the cases must be reamed. (I prefer reaming over turning.) Trimming isn't as much of an issue, but is still required every 2-3 firings with W-W brass and my 'mild' prefered loads. Other brands of brass require trimming after every firing, and reaming every other.
If you don't have experience with the hot .22s; don't let me get you down. They all have brass flow issues. They only people I've ever seen deny having to trim anything from .22-250 to .224 SuperMaximumUltraImprovedScreamingSquirrelCannon... are the guys that don't reload.
Like I said... I think .220 Swift will rechamber just fine. If not; .220 Swift Ackley Improved should be a consideration. .225 Winchester is just fine. Keep it if you like the rifle the way it is.