You need 6m silver to max attribute of Change. I think it is worth. JACKPOT mob is the Shining Silver mob. You can farm item easier from them (if the drop chance of this item is not too high, DPK < ~ 100 - 300). Something like Wizard Bracelet which has ~500 DPK hard to be dropped by JACKPOT mob. You only change SMALL mob.
JACKPOT mob usually drops the item of its previous mob. For example: 1st mob -> 2nd mob -> 3rd mob. If the 3rd mob is Jackpot, they will drop the item from 2nd mob. However, there are some exceptions, like 2nd mob can drops 2nd mob item or 3rd mob drops 1st mob item, but the percentage is low.
Moreover, sometimes you can get a MEGA mob, which is drop much silver (not gold mob or shining mob). This is a EXTREMELY BIG Raflower (the root in Crystal mine 1). I met it 3 or 4 times before.
Sometimes, you change a mob into an ET mob. They are fuking damn strong, can kill you in 2 or 3 hits, i was dead to them many times when i was in rank 6. You can change mob to kill bot if you are lucky. Moreover, you can kill bot or afk by using this trick: Summon a mob from red orb or blue orb mob which is high level like 200+, and Change it. This 200+ level mob will live longer and kill all low level bot or AFK