I did find out about Zygarde's forms being usable in Pokemon Sun & Moon.
Now, assuming that Zygarde doesn't have any EVs, the 10% Forme's base HP can either range from 50 to 65. Whereas, the Complete Forme's base HP can range from 212 to 227. This would mean that Complete Forme Zygarde's base HP surpasses that of Wobbuffet, and is only weaker than that of Chansey and Blissey.
In regards to Swarm Change, perhaps it would work as follows...
If 10% Forme Zygarde's current HP drops below 50%, it will transform into 50% Forme Zygarde. But if 10% Forme Zygarde's current HP drops below 25%, it will transform into Complete Forme Zygarde instead.
As for 50% Forme Zygarde, it'll probably lose the Aura Break ability and gain Swarm Change itself so that it can change into Complete Forme Zygarde if its current HP drops below 25%. This would probably mean that only Complete Forme Zygarde would have the Aura Break ability from here on out.
Swarm Change can't be Role Played, Traced, Skill Swapped, or nullified, ensuring that only Zygarde can have it.
Obviously, you want to take down 10% Forme Zygarde with a single hit if you don't want its ability to activate. But that can be almost impossible to pull off if Zygarde is holding the Focus Sash, which you'll probably see a lot of in Pokemon Sun & Moon.