Throughout the games, several Pokémon have been given multiple forms which they have the ability to interchange between. These forms were originally exclusive to the version they were introduced within but they have since been able to be on these forms within future games and Pokémon HeartGold & SoulSilver is no exception. Below are the Pokémon Forms and how to switch between them

#386 Deoxys

Deoxys was the first Pokémon in which to receive alternate forms. Originally these purely depended on the version the Pokémon was on but in Diamond & Pearl, it was made so you could have any of them, if not all of them, at any one time.

Like Diamond, Pearl & Platinum, Deoxys' forms are changed by your proximity to Meteorites. These Meteorites are set at the southwest part of Route 3. If you go up to them with Deoxys in your party, the Deoxys will change its form. To change to Normal Forme, use the top-left Meteorite. To change to Attack Forme, use the top-right meteorite. Defense Forme is obtained by using the bottom-left meteorite and finally, Speed Forme is found by using the bottom-right meteorite

#479 Rotom

Rotom's forms were introduced in Pokémon Platinum by using the WiFi Event item; Secret Key. However, in Heart Gold & Soul Silver, you do not need this item to access Rotom's forms

If you take Rotom to Silph Co. and have it at the front of you team, go to the Elevator and you'll be able to go up to it. Once there, Rotom will power up the elevator and take you to the room. In the room, select the respective item for Rotom's forms. You can change forms at will. To change Rotom out of that form and back to its base form, just select the tile where Rotom used to be

#487 Giratina

Giratina's alternate form was introduced in Pokémon Platinum as the form that Giratina takes in its home dimension. However, it was possible to keep this form outside of the Distortion World by using the item known as the Griseous Orb.

However, this item always returns to your pack when you bring Giratina into a trade. As such, the only way to get Giratina in its Origin Forme in Heart Gold & Soul Silver appears to be to receive the Giratina through the special Arceus Event. This level 1 Giratina will be holding the Griseous Orb. If you remove it, it will become the Altered Forme Giratina.

#492 Shaymin

Shaymin's Sky Forme was also introduced in Pokémon Platinum. This form is unique in that it is the only form a Pokémon can take that cannot be kept at all times. Instead, it is only available at certain points

To switch Shaymin's forms, you need the item known as the Gracidea. This item can be obtained from the Goldenrod Flower Shop if you show the woman the special Shaymin given away at events in 2008 & 2009 around the world. This is a Key Item so select it and then select Shaymin and its form will change. However, Shaymin can only be in Sky Forme from 5am to 8pm and will revert when frozen or put in the box.