Things You'll Need
Instructions
Attach a Game Boy Advance Link Cable to two Game Boy Advance consoles.
Start "Pokemon Emerald" on both handhelds.
Go to the second floor of the "PokeCenter" in any city. The lady on the far right of the room allows users to trade Pokemon between the two games.
Talk to the lady behind the counter on the far right of the room. A new menu will appear.
Select "Trade Pokemon" from the list of available options. The Pokemon in your inventory will appear.
Select Deoxys to trade the Pokemon to the handheld connected with the Link Cable. Once traded, the Deoxys will change forms.
Repeat these steps on the other Game Boy Advance handheld to trade the Pokemon back to the original trainer. The Deoxys will keep the new form it changed to when it was originally traded.