My Field report concludes that Shaymin-Sky form is obtained by levelling up a regular Shaymin to level 5 and evolving it like you would with any other Pokémon. This is how to change its form.
*Frantically tries to hatch his Sinnoh Eggs to be the first person to discover if this is true or not.
Has anyone been able to confirm if the form change is permanent, or can it be reversed? Currently testing the same thing with Giratina, and I'd very much like to be able to revert back to the Origin Form...
As far as I know how the system works you only can have one level set to "evolve" and it can be done any time post-reaching-that. So if both "evolves" at level 5 then it should be able to go back and forth because both will be at least level 5 EDIT: Considering Giratina doesn't from your report then maybe the point should be made so it can be- Its a very easy add on in my eyes
I made a thread here asking about the possibility of implementing a form changer in to the code. Tl;dr, it's a lot harder than it looks, and probably won't be around for a long time (Staff IS trying to find a solution).