I recently bought an 13.5 Trek Cali for my daughter (9 years old). I got an incredible deal on it and bought it sight unseen from an out of town seller. After getting it, I realized it had 29" wheels on it instead of the 27.5 the more current models have. I was actually looking for a 26" at the time, but it was such a great deal, I couldn't pass it up.
I like that it is something she will be able to grow into, but the tires are absolutely huge for her. I did some modifications (moved some spacers and flipped the stem) so it is definitely ridable.
I am curious if I could throw some older 26" wheels on the bike for the next year or two until she gets more comfortable with the bigger wheels. It has disc brakes so that shouldn't be an issue.
I know she is going to have a much lower BB that may lead to a few pedal strikes and it is going to look a bit akward with the longer seat tubes and forks. But since I am changing both wheels, it shouldn't affect the overall geometry too much.
So what do you think? I know it isn't ideal, but any reason it wouldn't work?
I have seen this question, but it addresses only changing the front wheel (which would change geometry. And also this question about going down to 27.5, but he is looking more for performance improvements.