Hello @Slokokaturt!! To change Rotom's form, you have to interact with a pile of boxes located in:
- Vermilion City Pokémon Center
- Celadon Department Store 2.
- Goldenrod Department Store 3.
- Lilycove Department Store 4F.
- Veilstone Department Store 1F.
When you change the form of your Rotom in the pile of boxes, it will learn the corresponding move (Rotom-Heat: Overheat; Rotom-Wash: Hydro Pump; Rotom-Frost: Blizzard; Rotom-Fan: Air Slash; Rotom-Mow: Leaf Storm).
You can change the form of the same Rotom from one to another.
The NPC you mentioned, it's a Tutor, who can teach your Rotom, for P12.000, the move it learns by changing it's form, that's why he said "that's not the correct Pokémon".
I hope this information has helped you!
Regards!