Are you dissatisfied with them? You could end up butchering a pair of perfectly good speakers.The internet is full of people who have opinions about everything (whether they've actually heard a given piece or not), but real experts are few and far between. Just 'dropping in' some driver is unlikely to provide much if any improvement. Different drivers=different sound. Unless they are very close in their electrical and mechanical properties, and you are prepared to re-engineer the crossover, you may end up with worse sound, not better.
One thing that might be worth doing- speaking of crossovers- is to contact Danny Ritchie (GR Research) and send one speaker to him and have him re-engineer the crossover. I have heard speaker designers say they spend more time getting the crossover right than any other aspect of a design. This could be worth pursuing. The downside is, even one (he only needs one to test, and the customer installs the xover in the second one) of these would be expensive AF to ship and you probably don't have the cartons after 15 years.
I would sum up my thoughts as: There are probably ways to improve these but the cost-effectiveness of doing so might not be good. If you still enjoy them after 15 years, they're probably not that bad, and sinking $$$ into 'mods' may produce something that isn't really 'better'- just different, and maybe actually worse. If it ain't broke, don't fix it, you know? If you want to upgrade, start over. Consider taking advantage of somebody's upgraditis by shopping aftermarket and then sell these to somebody starting out.
