@Wack yes you can convert gear from one class to another via Jasper but the MH/OH skills will convert to the new class. See chart below. For example, if you Jasper the Skyfall staff from Wizard to Warrior, it will convert from comet/meteor to a sword with flurry/throwsword.
Jasper conversion:
Wizard Warrior Rogue
Staff Sword Chakram
Wand Lance Flintlock
Sword Hammer Dagger
Gauntlet Axe Bow
Manshld Hatchet Mirror
Skull Horn Trap
Tome Totem Vial
Orb Shield Bomb
Regarding skill conversion. You can change the special skill only on MH/OH weapons with Amber. So you could be Warrior class using a hammer & change the special skill to meteor. However you can’t use Amber on eternal items.