You need a gracidia flower to change shaymin.
There should be a lady in the Pokemon center in Lacunosa Town who talks about the Gracienda flower. I'm sure that if you have Shaymin with you, she will give you the Gracienda flower. You might have to do this during the time that the Gracienda works, and Shaymin might have to be first in the party.
PS it needs to be the event shaymin with the fateful enounter in it's summary