What would be involved in updating Hersey II’s to Hersey IV’s?

If you get them at a reasonable price, I say live with them as is. Then after time goes by, see if you are really missing out on anything. Nothing wrong with enjoying something as is! Have fun! 

Since the Heresy IV isn't out yet and there are no specs on the Klipsch site yet (that I could find), it's a bit of conjecture.  I think I read in one of the threads about the Heresy IV that it will be a ported model, so I think you would have to do more work than it's worth to take a II to a IV. 

The II is well regarded, and many seem to like them better than the III.  You can buy aftermarket crossovers (or kits), and can replace most of the drivers.  Bob Crites offers great parts at reasonable prices.  There are some other options for aftermarket crossovers for Klipsch speakers designed to "improve" the sound (not sure about the Heresy II).  I have stuck with Crites parts.  I have upgraded my Heresy I by replacing the woofer, tweeters, and crossover, and putting Dynamat on the outside of the horns. 

As someone else said, give them a try stock, mine were very nice when I bought them, and they were about 40 years old when I got them.

Klipsch has forums with a ton of information about upgrades and mods.

I would think there are cabinet structural changes to be made

I am in the same boat: Here's what Bob Crites responded to me in an e-mail. "We did a little test a few years ago between a Heresy III and a Heresy II. The Heresy III was new at the time and of course the Heresy II was about 25 years old. The Heresy III was the winner in that test. Then I put a pair of our new Heresy II crossovers and a pair of our titanium tweeter diaphragms in the old Heresy II and test again. This time was not that much difference between the two. In fact some people preferred the sound of the rebuilt Heresy II.

So, with a pair of our new Heresy II crossovers and the titanium tweeter diaphragms, I think you are very close to the Heresy III."