Here's a headcanon for ya: the Rahaga were never turned back into Toa Hagah.
Interestingly, this change has virtually no effect on the storyline whatsoever. The Toa Hagah accomplished absolutely nothing from 2007 on. They stopped the kaiju fight in Xia, sure, and they served as vessels to introduce the Dark Hunter fleet, but that was about it. Otherwise, they've done jack squat.
Indeed, this change would make everything about these characters better. As Rahaga, they had interesting personalities, lots of history, individual masteries of Rahi capture... as Toa Hagah, they were nothing but recycled elemental stereotypes with new masks. Toa Iruini is Toa Matau, but with none of the charm. Toa Norik is Tahu without any character flaws. And none of the others even bear mentioning, since they've never been given any character features or personalities since the transformation.
Their transformation back to Toa let us learn about and see in use their Kanohi, and I consider that a plus.