you can use Jasper Crystal to change the Weapon of a Class to another Class. some Sets & Legend Affixes can’t be rolled with Crystals, so if you want something on an item that is for another Class, you would use Jasper to switch it to the Class you want.
this post shows what each Weapon can be converted to when changing Classes when using Jasper. hope it helps.
I just took a look in the Codex, and the Masochist’s Edge Bow was Jaspered from a Warrior Masochist’s Edge Axe.