I have a friend thats got 2 10" JL subs for sale. Would it be possible to to remove the 8.5" subs from the mach460, drill out bigger holes, and replace them with the 10" JL's?
No, the subs in the MACH 460 are 6x8s. I doubt there will be enough volume in that little plastic enclosure under the deck to be adequate enough for the JLs. ALso you have to take into account the Xmax on the subs. I'm sure they will be hitting the rear deck.
Just build an enclosure.