It's almost certain that any frame designed for 27" wheels will not accomodate 26" wheels. However, it will usually accept 700c wheels, which are only slightly smaller in diameter. The thing to look out for is the brake reach. If there is not enough adjustment, then the brake blocks will not reach the rims of the slightly smaller 700c wheels. You may have to buy extra-long reach brakes. Maybe the best alternative is to stay with 27" wheels. Good tires are still available for this size and you can even get reasonably good quality replacement wheels or rims in this size from Harris Cyclery if your present wheel aren't going to cut it.