Welcome Karen and Frisky,
You are now in the best place in the world to help you help Frisky. One thing most of will be pushing for it learning to home test, while this is critical until you start insulin it still is a very good way to see how well the diet change is or is not working for Frisky. It will also tell you much truer numbers than trying to go off vet numbers as all kitties ( even those that love their vets) are under stress at the vet's and stress raises BGs.
I would personally wait no longer than a week on a diet switch alone, before looking at starting insulin. While we have many kitties go into remission here, almost all of them needed at least some support from insulin to let their pancreas heal.
But should she need to go insulin it's no big deal, simply 2 tiny shots a day, with a few pricks of the ear to test her blood. Diabetes is so easy to manage that several of us have adopted kitties from this board, myself included, as diabetic.
Looking forward to learning more about you and your baby Frisky
Mel, Max & The Fur Gang