For Rotom in HG/SS you actually don't need the Secret Key to change Rotom's form. You have to go to the Silph Co. building in Celadon with Rotom as the first pokemon in your party, and go straight up from the entrance, to an elevator that was previously out of order, you will now be able to use it, it will take you to a room where you can change Rotom's forms like in Platinum.
For Pokemon HeartGold Version on the DS, a GameFAQs Q&A question titled "Rotom..?".
For Pokemon Legends: Z-A - Nintendo Switch 2 Edition on the Nintendo Switch 2, a GameFAQs message board topic titled "How do I change Rotom's form?".
How do I change Rotom's form? - Pokemon Legends: Z-A - GameFAQs
For Pokemon Legends: Z-A on the Nintendo Switch, a GameFAQs message board topic titled "Stereo Rotom".
For Pokemon Platinum Version on the DS, a GameFAQs Q&A question titled "Can some one tell me an action replay code to activate rotom event?".
Can some one tell me an action replay code to activate rotom event ...
For Pokemon Platinum Version on the DS, a GameFAQs Q&A question titled "How and where can i catch Rotom??".
For Pokemon Legends: Z-A - Nintendo Switch 2 Edition on the Nintendo Switch 2, a GameFAQs message board topic titled "Rotom Phone".
Rotom does not evolve this is what you do: 1.get secret key (already over) 2.defeat the boss at the team galactic eterna building 3.catch a rotom (room with the tv at old chateu 8:00 pm until 4:00 am) 4.go to the galactic building 1 floor and press A in the gap between the strange looking bookcase in the very left 5.select the form you want rotom to be in.