the 46t tooth gear is smaller than the 62t one and will will give you higher gearing (for the same pinion size), and one is made of plastic, the other hardened steel. Pretty sure thats most of the differences
You will probably want to change your pinion afterward. To keep the same gearing: if you want to go from 62 to 46 on the spur, you want a pinion as close to 0.74x the size of what you were running. To go from 46 to 62, you want it to be as close to 1.35x times the size of what you were running.