I would hate a lot of AH3 players moving to AH2 which could reduce Ah3 #s if the fight got really big in AH2. That's my only devils advocate.
I can't imagine that would be a significant problem.
First, you're not going to lose anyone interested in Scenarios/FSO. They may add it for an occasional hoot, but scenarios will be run on AHIII. I guess that's the other minor change would be to disable things like the SEA and H2H in the old version. But it's a LOT easier to rip stuff out than it is to put new stuff in.
Second, I could see a lot of AHIII players adding it on, I can't see them giving up AHIII and all the people they hang out with. Anyone who even thinks that may be a potential problem should think long and hard about whether the ball has been moved forward in terms of game-play or whether it has stagnated if you can't successfully compete with a 5 year old version of yourself. I think the vast majority of AHIII players would add rather than switch. Especially once they try it again and think, oh, these graphics really are a lot worse than I remembered.
My guess is you would draw in 4 types:
1. Old players who would come back and buy it just for the nostalgia kick. That might be enough of a spark to reignite their flame and get them thinking of maybe giving AHIII another try.
2. AHIII players who will keep their AHIII account, but buy into the AHII server for the occasional stroll down memory lane, especially if they see some old familiar names in there. And maybe they will start peer pressuring them to come to AHIII. Come back to join the current scenarios.
3. Kids may have money to buy a game, but who's parents aren't signing onto any subscription trap. Maybe AHII becomes the bush league breeding a whole new generation of customers.
4. All the other people who don't mind buying a value priced game to have some fun with, but hate subscription entanglements as much as they hate F2P grinding micro-transactions.
This seems like it would be a low cost, lowest risk approach to help out in some reasonable time-frame.
Should be fairly minor code changes, no new graphics or models needed. Set up the payment system and release it on Steam as a new SKU for $29.99.