I looked into this on my son’s 13.5 trek 4800, LBS put me right off, BB will be way too low, shorter cranks and forks, and totally mess with geometry, I was tempted to do it anyway as an experiment, then hey presto he had a growth spurt and 26-er was perfect, just turned the riser stem upside down emily batty stylee, looks like a mini 29-er.