I have some left over soft-body armor (Level 3A). What tool would be appropriate to cut it to fit into a new plate carrier?

None. You'd probably diminish its integrity. You'd probably damage the fibers and their weave wherever you cut it.

Don't cut it, it will lose structural integrity and begin to shred, as Kevlar ate simply loosely woven fibers. Fold the excess material to fit the plate carrier and then tape them tightly. It won't be perfect but it will be functional.

Small arms protective insert Plates go in a plate carrier?

I'm not sure about the armor you want to cut up, but almost all armor is designed such that it becomes unusable after the damage associated with protecting from the impact or puncture of the bullet. It no longer becomes usable. I imagine cutting up some armor to put into the plate carrier would effectively render the protective properties useless. Plus, what LTC Jason Mackay said.

in my plate carrier I had just plates while deployed, less weight on my shoulders while climbing afghan mountains

Uh don't cut it because it will probably ruin the integrity of thesoft armor.

Soft body armor loses effectiveness as it's worn over time. It might not be any good anyway.

Cut it?