I've heard that ELC's will eat old seals, but cannot confirm or deny this.
I'm running Caterpillar ELC in my truck and have been since I redid the head gaskets and oil cooler seals in 2004. At that point, essentially every gasket or seal in my cooling system had been replaced, and with one exception, I haven't had any problem with leaks. That exception is with the plumbing for my coolant filter...I used teflon tape on the threaded fittings, and I found leaks from these joints shortly thereafter...I guess that ELC eats teflon tape and you'd be better off using pipe sealant on any threaded fittings. Other than that, I've been happy with the ELC...my radiator still looks brand new inside (I replaced it at the same time I did the ELC conversion), and I haven't had an opportunity to look inside my cooling system since the conversion, but AFAIK all's well
So, I guess the best answer is, if you can redo your coolant seals beforehand, it would be good insurance against having a problem further down the road...whether or not it's necessary insurance, I do not know. Also, keep in mind that you will still have to check ELC for protection levels...the additives that you have to manually add to the green stuff is already in the ELC, but it will still lose effectiveness over time (albeit IIRC it lasts longer than the additive added to the green stuff). Caterpillar even sells an additive that's meant to be added to the ELC about halfway through its service life.
This probably created as many questions as it answered, but I hope it helps a bit...good luck!!