Might want someone to cross-confirm but this is my understanding.Also, I have an Adamant 50% Ziggy in Gen 6; do we have any idea whether Zygardes transferred from Pokebank can change form too?
All Zygarde from pre-gen7, and those created in gen7 with less than 100 cells (in other words, all with Aura Break) can be considered "loose" Zygarde. All loose Zygarde can be disassembled into cells (completely destroying the pokemon) and you can use the cells to generate another Zygarde (new IVs, nature, etc). Loose Zygarde cannot change forms without disassembly.
When you have 100 cells you have the option to make a "solid" Zygarde with Power Construct. This one can change forms and cannot disassemble.
If you transfer Zygardes from bank they can be used as a "free" +50 cells basically.