I have a nice colnago mountain bike w/26 wheels and disc brakes; am thinking of changing to 700 wheels as i am 6'3" and bike feels somewhat small. would this negatively affect ride, handling, geometry...? i tried on 27.5 wheels from other bike, and they do fit.i am just an older recreational rider and am wondering if this would work . thanks guys
So long as you can get the wheels in the frame with enough clearance and have the brakes work, you're OK.
Note that your proposed wheel swap may not give a significantly larger outer diameter of the wheel+tire (which if increased would make the bike a bit taller riding).
Now to what I think is the answer to your real question: If the bike is feeling small, the wheel size is one (minor) portion of it; the rest of the geometry (top tube length, seat tube length, stem length, etc.) have a far larger effect than the wheel size has. So, you may need to switch frames.