For me it's about someone pretty and outgoing and confident and successful becoming meek and timid, relegating them to something of a societal outcast from then on.
I really don't like nerd tfs that are super over the top, with the transformed person becoming hideous. Nerds can still be cute, and I don't mean in a Belle Delphine wearing glasses sort of way
But it's their attitude changing that's the real clencher in the transformation for me. Going from pretty to plain is ancillary to becoming shy, cripplingly insecure and socially awkward. For me, the best nerd tfs lean on that aspect.
Someone wrote a fantastic life/bodyswap story on Deviantart where the pretty popular girl has her life, her body, her boyfriend and her body swapped bit by bit by a shy, chubby, socially awkward Swiftie.
Another author wrote a TG where a failing student unwittingly enters into a deal to become his teacher's assistant for a chance at better grades... and ends up suffering a unforeseen twist in the deal.
[spoiler]Despite now having massive intelligence, she's rendered too anxious and insecure to pass any tests[/spoiler]