I have a used Trek T900. My 8-year-old daughter and I ride it mostly on the local gravel and paved bike-paths. Every once in a while we ride a little single-track.
The bike came with 26" wheels/tires.
I'm interested in 29" wheels for a smoother ride, and I'd prefer big hydraulic disc brakes to the rim brakes.
A 29" wheel/tire is impossible in the rear, obviously, and I'm not ambitous enough to add a caliper mount in the rear, either.
On the other hand, it occurs to me that I could easily replace the front fork with a disc brake-ready rigid 29" fork. Then I'd just need a wheel/tire and a front brake.
I understand that it would slightly change the geometry, but that seems insignificant.