You can deconstruct a Zygarde made of less than 100% of the cores and cells, but according to Serebii, you can't deconstruct one made of all of them (a power construct one).
And even if you do deconstruct your level 100 Zygarde from gen 6 (not a power construct one) when pokebank gets updated, it won't be level 100 anymore, even if you can put it together with more cells to make it a power construct one. (When you deconstruct a Zygarde, it forgets all the moves you taught it, loses all the EVs, all experience and levels, and it re-rolls it's nature and IVs.) So it doesn't matter if you can or can't deconstruct a gen 6 Zygarde. I think you still will be able to though, since you can deconstruct other people's Zygardes.
So you might as well make this Zygarde, because you can go to the Zygarde lab, save in front of the machine and make a power construct Zygarde, and if it isn't adamant, you can keep soft resetting until you get an adamant one.
