Whatever this is, I don't think we should be thinking of it as a simple form change. Both times they've done something like this before, they brought something new to the table with them.
Giratina brought form changing.
Kyurem brought - and is still the only pokemon capable of - fusion.
Whatever Zygarde will be doing, it's likely something entirely different then what we've seen before. It's not too hard to believe this little green blob is somehow related.