JuicyJJ,
Welcome to the forum.
Cornwall is an excellent speaker and your instincts are good in thinking about updating the CW.
As mentioned in posts above, the configuration of the CWIII does not allow for upgrading an older CW to CWIII.
Sounds like you are still in the acquisition phase. My suggestion would be to acquire a pair of original CW's. I think these updates would work better than with a CWII. .
There are a couple of reasonably priced updates you can do which will leave you with a killer speaker that I'd put up against a CWIII any day.
The CT-125 tweeter is a drop in replacement for the K-77, which is in the original CW. Bob Crites sells these tweeters and they sound very good. IMO a huge improvement over the venerable K-77. (Bob is BEC on this forum)
The age of the crossovers is an issue, but they can be restored, essentially to factory spec, with fresh capacitors. Bob Crites is a good source for parts or turn key rebuild if desired.
If you want to go beyond that, it is a small crossover mod to cross the tweeter/mid at 4500hz which will smooth out the upper mids nicely.
One more thought on the cool factor of the original CW's. These use the K-77 tweeter and K-55 mid which are retained now only in the higher in LaScala and Khorns. The original Heresy and CW's use these drivers, but they are too pricey now for Klipsch to use them in the current manufacture Heresy and CW.
Food for thought.
"I know I'm paranoid.... but, am I paranoid enough?" --Tom Clancy