It's possible: before height X calculate hash with old function, after height X - calculate hash with new function. The reasons why other altcoins don't do it:
(a) hard forks are very annoying and risky, especially when the coin is already widely adopted
(b) most altcoins don't have devs competent enough to do the change.
Mooncoin is still in its infant stage, we're forced to do a hard fork soon, and we have Deacon who's able to do the change (the complexity of the change is not higher than switching to gravity well). So I think we should consider it.
Question for anyone who knows...Why is a new wallet considered a "hard fork" in this case? I have not changed the cryptographic keys used for the currency. As far as I understand, if you are a regular user and upgrade your client, once the block height gets to the height specified in the code for using the grav well, everything just keeps humming along as intended.
The only people who will get left out are those who keep running the old code.
Is that not correct?
Yes, I believe that's correct.