I am attempting to do this during the day (1:00 PM) by using the Gracidea on Land Forme Shaymin only to get the message: "It won't have any effect." I've attempted this with 4 different Shaymin, both a normal and fateful encounter in either Forme. (I read that Gracidea only works on event Shaymin.) For the ones that were automatically in Sky Forme, the following happened:
-I opened up the PC.
-Shaymin's summary popped up. It's lower screen icon, 3D model and ability are all in Sky Forme.
-I moved it to the Battle Box, thinking I could battle with Sky Forme as long as I don't remove it from the PC.
-Its model and ability reverted to Land Forme. The icon was still in Sky Forme.
-I went into a different box and then back.
-The icon had reverted to Land Forme.
Is it a matter of the PIDs and extra bytes? I honestly wouldn't know...