To evolve your Porygon2 into a Porygon Z, you have to trade your Porygon2 to a friend while it is holding a Dubious Disk.
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practically the same way as you evolve porygon only now you have to use the dubious disc which can be found on the other side of the big lake on route 225.
Make porygon2 hold a dubious disk Then trade it in order to evolve
Porygon2 evolves by trading with a Dibious Disc
You have to trade with it holding the dubious disk. This can be done when porygon2 is at any level.
To evolve it into a Porygon2, trade Porygon holding an Up-Grade. To evolve Porygon 2 into Porygon-Z, trade Porygon2 holding a dubious disc
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