Unless there is more to it than a flip chip for the rear end, you're going to have a dramatically different bike when you convert - lowering the BB enough to compensate for a 1" larger radius wheel just by moving the rear axle upwards is going to cause you to have a head angle several degrees slacker. Maybe you can adjust the travel on the fork as well?
The big issue with all of the conversion ideas I've ever seen (and I've thought about building some myself) is that by the time the kid is big enough for the next wheel size up, they really need a longer cockpit/reach/front center as well. You can just slap a longer stem on, but that's far from ideal.
-Walt