Wow, that is actually a really good storyline. I mean, it totally removes the plot holes caused by the movie. Unfrotunately, i am not quite sure if they are able to convert back to their Believix or not, probably, since after all we have seen some errors, plots holes etc, which could totally get us confused! Plus, in the movie, Daphne was still a nymph, she wasn't healed and the Winx totally did not talk about anything connected to Sirenix. Plus, their outfits were all civilian clothing from season 4. So, i would say your idea is great, but it creates huge plot holes, further confusing us, so maybe Nick will just pass off the movie as a special or promo or something....