Details are here: Raid Battles and New Gym Features are Coming! - Pokémon GO

Highlights: Gyms have a fixed capacity of 6 now and you can't add a mon that's the same species as something that's already in there.

The order of attacking is now based on how long a mon has been there as opposed to ranking by CP.

Instead of prestige, individual mon now have a "motivation" stat that decreases over time and can be restored by giving it berries.

Annnd, it looks like they're compensating for the potentially lower capacity by increasing the number of gyms. I can see two gyms from my desk that used to be regular stops.

I am intrigue!

The part where all stationed mon get kicked out of gyms has been happening. Weirdly, it's been gradual. I saw the number of things I had defending gyms hit 5, then 3, then 2, and now 1.

I'm kind of going nnnngg, be done already, cause I'm thinking the booting process is likely to be one of the last steps to the rollout.

And I live in a town where damn near everyone is Mystic and I'm genuinely concerned that all the gyms could be full and stay that way if I don't get the jump on it.