I have 2 turning 5200 packs could I change out the leads to 12 gauge instead of the 10 gauge they came with?

No reason why not if you are proficient in soldering

For what reason do you want to do it ?

Cheers

Trying to put in slider and hard to get the center body on.

You can, but you need to be VERY good at soldering. one slight mishap can lead to a fire/explosion very quickly if you create a dead short.

Where do you live? i'd be willing to repair them for you, free of charge.

sorry was wrong there 8 gauge wire.