you have to talk to mr backlot in the Pokemon mansion south of hearthome city after receiving the national dex and beating the elite four if he says a porygon came up to him you can look for porygon until you find one in the trophy garden out back
PS. you only have a 2% chance of finding it in the trophy garden at any time
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The Up-Grade evolves Porygon into Porygon2 =).
It is used to evolve Proygon2 into Porygon-z.
In Pokemon Platinum (I don't know about Diamond and Pearl), Porygon is located in a house directly behind the Pokemon Center. If you want to evolve into Porygon 2, trade it holding a Up-Grade, and if you want to evolve Porygon 2 into Porygon Z, trade it holding a Dubious Disk. I do not know where htese items are found. If you want to get Porygon the harder way, use Pokecheck.
give porygon (or possibly porygon 2) dubious disc ( i think you nick it from galactic hq) and trade him.
Porygon-Z is a Normal type pokemon.