Check the ingredients of the pellets. Most pellets have basically the same ingredients so it may not be as critical to switch them over gradually. Whenever I've tried mixing pellets as recommended, they only pick out the new ones if they like them better anyway, so switching gradually has never really worked for mine.
I am not hearing that you've offered her any wet herbs or salads, or otherwise gotten more fluids into her as I've been urging. Since she's taking an antibiotic, hydration is even more important now, since antibiotics will slow down the gut and decrease appetite, as well as dehydrate. Keep checking her weight, stomach and poos.
I'm sure your regular vet just doesn't want to give advice without seeing her. I would schedule an appointment as soon as possible and see if they will give her subcutaneous fluids.