An interesting quirk came up in my game last night. In the end it was inconsequential as to the death of a white dragon (at least I treated it as such), but now I'm curious about the interaction, for future reference.
Frostburn
In addition to coldfire, certain weapons and spells of ice and cold can deal frostburn damage, burning the tissues with extreme cold. Like normal damage, frostburn damage results in the loss of hit points or ability score points. Unlike normal damage, however, frostburn damage does not heal naturally and may only be healed magically with a successful DC 25 caster level check as long as the victim remains in cold or colder temperatures. If the victim reaches an area of moderate or warmer temperature, his frostburn damage becomes normal damage that can then be healed naturally or magically in the usual manner. Spells such as control temperature become invaluable when encountering creatures that can deal frostburn damage.
A creature’s resistance or immunity to cold also applies to frostburn damage. This includes characters protected by spells such as resist energy [cold] and protection from energy [cold]. Endure elements spells and effects confer no protection against frostburn damage.
Creatures vulnerable to cold take +50% damage from frostburn.
Okay, so normally a white dragon would be immune to the damage of a spell that dealt frostburn damage (such as Frostfell).
However, if the Frostfell spell was treated with the Energy Substitution feat to instead deal Electricity damage...
Does it deal electricity damage, which would affect the white dragon, but no longer has the "can't be healed in cold" clause, since it was changed from frostburn damage to electricity damage?
Or does it do electricity damage, and still retain the "can't be healed in cold" clause?
Or does it do electricity damage, but a cold subtype creature is still immune to the "can't be healed in cold" clause?
Is there some other legitimate interpretation I'm not thinking of?
I treated it as the second option, though the dragon never had a chance to heal such that it would make a difference which of the above interpretations I had used. But, now I'm leaning toward the first interpretation, because you changed the damage type to electricity, so it is no longer frostburn, and thus doesn't maintain the properties of frostburn damage.
Choose one type of energy (acid, cold, electricity, or fire). You can then modify any spell with an energy descriptor to use the chosen type of energy instead. An energy substituted spell uses a spell slot of the spell's normal level. The spell's descriptor changes to the new energy type—for example, a fireball composed of cold energy is an evocation [cold] spell.
Frostfell does has the [Cold] descriptor, so you can change it to use electricity
instead, meaning it is no longer frostburn damage, and thus has none of the properties of frostburn damage.