Only 10 Pokemon can change types, by different methods.
- Arceus changes type when it holds a plate. If Arceus doesn't hold a plate, it is Normal Type
- Castform changes type depending on the weather. Under Sunny Day it is Fire Type, under Rain Dance it is Water Type, under Hail is is Ice type and under Sandstorm it is Rock type. If there is no weather it is Normal Type
- Porygon, Porygon 2 and Porygon-Z can change type by using Conversion. This changes the users type to the opponent's type
- Porygon, Porygon 2 and Porygon-Z can change type by using Conversion 2. This changes the users type to the type of the last move that hit the user
- In Generation V, you can change Rotom's type in a room in Shopping Mall 9. Rotom will always be part Electric type but its secondary type can become Grass, Ice, Fire, Flying or Water when you interact with a certain item. If no item has been interacted with then Rotom's secondary type will be Ghost. In all previous generations, Rotom's type is always Electric/Ghost regardless of its form. The only thing that changes is one move
- Shaymin becomes part Flying Type when it holds a Gracidea Flower and changes to its Sky Forme. If it is not holding a Gracidea, it is just a Grass Type in its Land Forme
- A Darmanitan with the Zen Mode ability becomes part Psychic type when its HP is less than half. Its form and stats will also change. A Darmanitan without the Zen Mode ability stays Fire Type.
- When Meloetta uses Relic Song, it changes form and becomes Normal/Fighting type in the Pirouette Form (the red hair) and Normal/Psychic type in the Aria Form (the green hair)
- Kecleon changes type to become the type of the move that last hit with its ability, Color Change
I suggest Ign.com, gamespot.com, or supercheat.com
Some codes don't work though, if the code has D2000000 00000000 then it will work
there is also www.gamewinners.com it has HEAPS of cheats 4 Pokemon diamond and pearl and nearly every single other game as well.
You can buy either a Gameshark or Action Replay then search for codes online to get those items.