I've seen it done. Not sure just how much is involved to do it though. One thing for sure, the rear dropout spacing on the frame will have to be increased to accomodate either a multi cog freewheel, freehub, or one the newer 7 or 8 speed internal gear hubs. Do you have 24" wheels now? I wonder if the modern 26" mountain bike wheels would work in a Schwinn frame designed for the old 24" tires. The mountain bike wheels are much smaller than the old 26" that were used back then(559mm bead seat diameter vs. 597mm). I don't have the figure for the old 24" right at hand but it is on Sheldon Brown's website.