After hearing leaked song, something have been bothering me. I have a feeling that synopsis and song lyrics tell us more about up coming episode than it's seems at first glance. I have to write my thought down, before I can calm down, so here I go.
First, my ideas who got who's cutie mark:
Applejack -> dress making
Rarity -> weather
Rainbow Dash -> caring animals
Pinkie Pie -> Sweet Apple Acre
Fluttershy -> "party"
Now, in synopsis it said "The only way she can reverse the spell is by writing her own magic", and in song we hear how everything is turning ok. BUT, for me it doesn't sound like Twilight is using her magic to turn everything fine (like memory spell in The Return of Harmony). And if all they need is Twilight's magic, then what purpose does it make to say "We need your help"?
For me it seems like when their cutie marks and destinies switched, they got jobs they aren't good with, like Applejack making dresses. In song Twilight gathered her friends together in specific order. First is clearly Fluttershy before song started. Next is Rainbow Dash, who had Fluttershy's job. Then Rarity with Rainbow Dash's job. After that Applejack who is trying to make dresses. Last one was Pinkie Pie in farm. In other words, they always helped pony that original job is one a next pony currently have.
My idea is that they don't use just spell casting magic to help everypony, but rather magic of friendship I know, what a shock. It's looks like everything turns out fine every time friends got together and I assume it turned out fine because everypony helped pony in trouble to do her current job. This would go fine with other season 3 episode Larson wrote, Magic Duel. There Twilight learned to use her friends help while using her magic.
But how all of this fit with Twilight becoming alicorn and how they managed to "switches the Cutie Marks and destinies of all her friends"? Didn't Twilight said in Call of the Cutie that not even magic can help fillies to get their cutie mark any faster. One might think you need really powerful magic to do such reality altering magic. Maybe it would be possible if they have the most powerful magic known to ponydom --- Wait a minute! What if for some reason mane 6 used the Elements, and then by accident Twilight "switches the Cutie Marks and destinies of all her friends". After all, I would be interested on the Elements, what you can do with them and what not.
Now the alicorn thing. All I'm going to write here is pure speculation and should threat as such. So far most poster have threated her becoming alicorn as if she's turning to completely different being, getting more power than any other her friends. But maybe we have watched it in wrong way. Why are Celestia and Luna alicorns begin with? I can't remember where I read it, but I remember Faust saying Celestia have horn and wings to represent all pony races in Equestria, earth ponies, pegasus ponies and unicorn ponies. My though is that Twilight turning to alicorn is mostly symbolic, showing that with help of her friends (earth, pegasus and unicorn ponies) she can now achieve any goal. I'm NOT SAYING that she'll become any super-goddess. I'm sure she still need her friends help. Her transform is probably physical mark of her becoming one with her friends, ending one of her story arc. She's not anymore learning to be a good friend, she is now a "True, True Friend". And BEFORE YOU REPLY, I'm not saying this had to happens. I still think they could and probably should kept Twilight unicorn and than alicorn Twilight is Hasbro mandate, and this is just my idea of how this transformation can be justified in story lines and reasons.