well, actually, bb hight is measured from the floor, its just that us trials rider never really have a bb under axle level, so we just measure it from the axle. If you put 24" wheels on a 26" frame, the bb WILL be closer to the floor, unless you put big 2.7" ballon tyres on. I wouldnt really worry about it though, the only time it would matter if you were to put 24" wheels on a bike with an already really low bb (normal xc type bikes).