There was a Cannondale a couple years back that had interchangeable 26" and 700C wheels. 26" wheels and fat tires come out about the same outside diameter as 700C wheels and narrow tires. The Cannondale had disk brakes, though.

I think it's do-able, but brake fitment is the real bugaboo. Without seeing your existing bike, I can't say how easy it would be to adapt different brakes.

If you're after a good road/touring bike, I think putting narrower, slick tires on your 26" wheels will get you 95% of the way there at far less cost and complication.