How do you switch Pokemon in Game Boy?

First off you have to insert the GBA cartrige into the DS slot, then go to the menu ( the one where it says "Continue game","new game" and so on. then it should say migrate from the GBA Pokemon game that you inserted, then when you select that option it should say some thing about time then you have to select 6 Pokemon from the game that you wish to migrate (YOU CAN ONLY MIGRATE POKEMON THAT ARE IN YOUR STORAGE BOXES. SO YOU CANT MIGRATE IF THERE IN YOUR PARTY). Once you do that you need to go to Route 221, when you reach route 221 you have to go inside the building and talk to the man then he will let you out into the feild and catch the Pokemon that you migrated

HOPE IT HELPS:)

PS: I think you have to defeat the elite 4 and the champion before you can go to route 221