Abilities » Energy Trans
Can you "Energy Trans" or "Dark Trance" a Rainbow Energy, Multi Energy, or any other special energy cards?
As long as they are providing the proper energy type and don't violate any other restrictions written on the card, then yes you may.
Source: PUI Rules Team (2004-05-06), PUI Rules Team (2005-11-17)
Abilities » Energy Trans
If Porygon2 has changed a Grass energy to Lightning, would it be fair to say that Venusaur could not Energy Trans it (for the rest of the turn)?
Correct.
Source: WotC Chat (2002-01-17)