Shouldn't be a problem. I just replaced 7 cassette 26" wheel with a 5 cassette 26" that was lying around and it fits/works fine although the v-brakes had to be adjusted with the tighter fit.

And I also did that with a 27" wheel,replaced it with a 700c ($50 at LBS) which was a tad wider and had to bend/pull the fork a bit to get it to drop.

craiglist, pawnshops, LBS, REI, nashbar, ebay etc...