Depends a bit on the severity of the changes.just wanted to say that and ask if you want to do an 'event' in game that changes Sirens powers ..., or just kinda gloss it over
The temporary power feat lasts for the duration of the encounter or until you choose to stop maintaining it, whichever comes first. This includes turning off the power or switching to a different Alternate Power.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.