Jet2's most likely course of action will be to wait for an airline to dispose of a couple of B767's at the same time. This will ensure commonality and also makes the transition much easier. Also, given the age of the B767 it will most likely be in a lessors / financiers interest to sell on the aircraft rather than enter into a ne lease agreement. This would also fit with Jet2 previous history of generally avoiding operating leases and instead going to finance leases or simply mortaging the aircraft.
We've already seen this kind of block purchase with Allegiant picking up a block of the ex Thomson B757's from AerCap.