Winx Club-What happened to Tune and Digit?

SPOILER WARNING FOR WINX CLUB!!!!!!!!!!!!!!!!!!!
Tis the age old question.(Age old= Since 2013)
What happened to Tune and Digit?
These two pixies were supporting characters in the Italian cartoon Winx Club from their first appearance in the first episode of season 2. After the pixies disappeared from the series for the majority of seasons 4 and 5, most of the pixies returned for the rest of the series. Tune, Pixie of Etiquette and Digit, Pixie of Nanotechnology were missing.
And in their place were imposters!

These are those imposters.

Cherie and Caramel appeared alongside the main group of pixies in season 6. They are presented as the bonded pixies of Musa and Tecna respectively and all the main characters act as if they had been there all along. The viewer however knows something is wrong.
Where did these two imposters come from?
What have they done with Tune and Digit?

Okay, So in all seriousness let's talk about why Rainbow (or Nick) may have chosen to switch the Pixies before we get into all the conspiracy theories I've made up on the topic.
Cherie and Cara first appeared as main characters of the Winx Club spinoff series PopPixie. Tune and Digit were characters in this series too (although they changed Digit's gender and Tune is now the Pixie of vocal power for some reason for some reason). The only logical reason I can think of for switching Tune and Digit for Cherie and Cara is to promote PopPixie. Of course they could have included Cara and Cherie as bonded Pixies for the two fairies predominantly  featured in the series that don't have Pixies yet; I'm talking of Daphne and Roxy. Fans have been speculating about Roxy's Pixie literally before she even appeared in the show so it would make loads more sense to introduce one of these two as her's instead of replacing two exsisting Pixies for no reason.

First let's talk about Pop Pixie. The spinoff takes place in Pixie villiage and features various Pixie characters from Winx club. However none of the Fairy characters are ever mentioned in the series so it is safe to assume PopPixie at least takes place before Winx Club season 2 (when the Pixies first appeared). This would also explain the changes to certain character's personalities, especially Tune.

So what happened?
Well for one thing this show is not very good with continuity. There are several instances in the series where events are retconned or simply ignored for the sake of continuity. This is especially true in the series co-produced with Nick since they are...Let's be honest, pretty sh!t at writing a coherant story. Now you could just say that Cara and Cherie are just relics of a time when Nick had a say in the story of Winx Club and a lasting reminder to never let the people at Nick tell you how to write your series, however that's not why you clicked on this blog post is it?

Time for some Crack Pot Conspiracy theories!
Pixies want to bond with fairies. When they learn the last fairy of earth is coming to althea, they are all super excited. Perhaps Cara and Cherie grew jealous of Tune and Digit and decided to take their place. But how?
Time Travel!
So it's not like Tune and Digit disapear completly after Cherie and Cara's indroduction; they can be seen in the background in some episodes so they definately still exist. So something must have happened between season 4 and 6 to create the switcheroo. As mentioned earlier from season 4 onwards the show's canon gets rather... confused.
For one thing the Nabu dies in season 4 and then appears in the second movie which is supposed to take place after. In the same movie the Winx use their believix powers despite the events of season 4 implying they could only be used on earth. The circumstances of Daphne's "death" are changed completely in season 5 for the conveniance of the story.  So here's my theory, all the messed up canon from season 4 is caused my Cara and Cherie travelling back and forth in time trying to create a world where they are able to take Tune and Digit's places.

Attempt 1 
So they went back in time for the first time at the end of season 4. They did it on a whim so they ended up making several random, haphazard changes, creating a world where Nabu never died, Roxy never goes to Alfea, Love&Pet never happened, Helia never cut his hair, the Trix either never got captured after the end of season 3 or escaped very soon after and the Winx continue using believix after leaving earth. Unfortunately (for Cara and Cherie) even after all these changes, Tune and Digit still found their way to their destined fairies. C and C decide to change everything back to normal and try again...
Attempt 2
Next C and C decided to go back and prevent the pixies from ever meeting the winx by stopping them from being captured by Lord Darkar in season 2. This created the world season 5 takes place in, where the Pixies are never seen or mentioned once. Lord Darkar had an important role in the destruction of domino and Daphne's "death" so the changes C and C make are what caused the changes to the canon in season 5.
Attempt 3
This time C and C need to find a way to get close to their targets. With Tune and Digit out of the way, C and C are free to bond with Musa and Tecna. The other Pixies end up meeting with their destined fairies without any problems and know one except C and C know how hard they worked to create a universe where they can be the lucky Pixies to get a bonded fairy.

So yeah it's a bit of a long shot and this theory is really just for fun but... I don't trust Cara and Cherie, they are clearly up to something.
Feel free to share your own crack pot theories in the comments!