Keeping a permanent A/B def is, in theory, possible. Problem is once new changes are adopted, they have to then be specially made for one and the other version, which can then lead to smaller or bigger issues when something is forgotten in a rush to release an update.
Keeping "old" and "new" players happy is a challenge with every change, there really is no way around it. But, since i largely have nothing concrete to share yet, we will have to wait for more.
I really can´t speak much yet about the change, or what will change. I shared as much as i could (was able to, better said) with Cynthia trough the support.
So more will come when i get a green light, and more information in general.