As Pokémon Legends: Arceus changes the series formula greatly, the game also shakes up how trainers evolve Porygon and the rest of its evolution line. In past games, players would have to trade the Pokémon while it holds items in order for it to transform, but Legends now cuts the trading part out entirely. In its place, players only need to find two items in order to get Porygon to reach its final evolution.
Porygon will need to be given the Upgrade item to in order to evolve into Porygon2, as Porygon2 can evolve into Porygon-Z by using the Dubious Disk. Although these are two separate items, it is probable that you will earn both of these at the same time, as you can discover them in the same areas.
Upgrade and Dubious Disk can easily be obtained by buying them with Merit Points at Jubilife Village’s Trading Post (located on the left of Galaxy Hall). Upgrade will run players 1,000 Merit Points, as Dubious Disk costs 1,400 Merit Points. Of course, you’ll need to collect satchels all around Hisui to get Merit Points, and each typically gives you around 100 when found. As shown below, you can zoom into each area’s map to spot satchel locations, as they are marked by white rectangles.
If there are not enough satchels around to afford Upgrade or Dubious Disk, it is also best to check inside Space-Time Distortions for both of these. When you beat Pokémon inside of a Distortion, there is a higher rate of them dropping valuable items than they otherwise would in the wild.
If you’ve nabbed Upgrade or Dubious Disk, you are then free to drag it over from your satchel and onto the Pokémon for them to evolve.
Related: How to evolve Vulpix into Ninetales in Pokémon Legends: Arceus
