What if I pull the horns and xovers out of our 4731's and build some 722 size cabs with said horns and a pair of Eminence Kappalite neo 12's? Could sell the JBL 12's on feeBay to partly offset the cost of the Kappas.
You probably couldn't re-use the crossover as they should be optimised for the original 12" drivers - any other 12" will have at least slightly different frequency response and impedance curves, so you'd end up with a skewed response. If bi-amping, that wouldn't be a problem of course.
If you do go for the Kappalites, you probably want the 3012HO rather than the 3012LF you mention in a later post - the HO will get louder on less power, unless you plan on running without subs in which case you'd want the extended range of the LF model.