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If im understanding your question right then the answer is no…… Example AUmbreoncan be obtained inPlatinum, but its original region wasJhoto. i can still migrate thatUmbreonto my pokemonBlackeven though its in theShinnohRegion.
Source-Personal Experience
If you mean you migrate a pokemon from one region can you migrate it again the answer is yes you can still migrate it again…….Example i have aCharmanderon myFireredand i migrate it to myHeartgold, later i can migrate that sameCharmanderto my pokemonBlack.
Source-Personal Experience
No as long as it has no illeagle hidden machines in b/w and isn’t holding an item they can be migrated fine as long as its not from a gba game.Source: experience.