I have an idea that maybe in a story like this the formerly thin one's time spent in a fat body forces them to realize that they were secretly FFA all along, and all the teasing/meanness towards fat girls was just a "doth protest too much" type denial/immature bid for attention a la a kid being mean to another kid they actually "like-like". And similarly, the one who was originally fat realizing that while they may enjoy the increased attention and social rewards of being thin, there are actually aspects of living in a fat body that they love and miss. Cue fatty-turned-skinny deciding to embrace her curves and ditch dieting after the switch back, while skinny-turned-fatty may or may not vow to eat her way back to her former fat foe's weight once she's back to her slim self (possibly with said former foe's help). May involve varying degrees of the two developing not only a friendship but also a feeder-feedee and/or romantic relationship.Take for instance, the role reversal. If you're not familiar, it's where two characters switch their body weights(For our intents and purposes. Often, lifestyles and attitudes also change). There's always that point at the climax of the story, or even the ending where both meet up again. Normally the one who was fat and is now thin makes fun of the now fat one. But there's so much more that can be done with that interaction. Who's to say their attitudes would go like that? Would be it be possible to have it work out? Perhaps each girl tries to alleviate their current state before the interaction, making the drama there come from their internal thoughts of themselves and the other. I could go on and on, but I think you get the idea.
And, playing off the "Freaky Friday" plotline, this would be the theme song: