Yeah, the title basically give it away but here's the way to change these pokemon's form:
Deoxys: It's really the same thing as in D/P/Pt with the meteors and all . . . the meteors are East of Pewter City, you can't miss them
Rotom: Take Rotom to the Silph Co. Building (Saffron City) and have it first in your party. Go up the elevator (yes, Rotom will power up the elevator to get it working again) and you will see 5 electronic appliances. Talk to one and Rotom will change into that form
Garatina: The only way to change this is by getting Garatina from the Arceus Event cuz it will be holding the Griseous Orb so it will already be in Orgin Forme.
Shaymin: Show the 'Little Girl' in the Goldenrod Flower Store the special Shaymin available in 2009-ish and she will give you the Gracidea Flower, which is in the Key Items in your bag. Select use and use it on Shaymin and it will turn into Sky Form!! It won't be in sky form at night or when it is frozen though . . .
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