From what I understand, there are going to be not one, but TWO transformations: sirenix and harmonix! It seems that the first half of season five will air this fall and go on until spring where we will see the girls achieve the harmonix transformation. Serenix will appear in the latter half...but, the "ability to use their powers under water?" Um, can't they do that anyways? The water on Andros did not really stop them in season three.
There will also be a Christmas special for this year. Hmm...I'm guessing that it'll just be a winter holiday special because Magix doesn't exactly celebrate Christmas or have winter for that matter.