Switching feed

Hello guys

I was hoping to get some comments on experiences with switching foods cold turkey.

My chinchilla recently decided to stop eating pellets. We ran her through the vet, got her teeth checked out, stomach x-rayed and so on. Turns out the sneaky rat was just protesting her current food (had it for 3 years) and the vet said she just seemed plain bored of it, as weird as it may sound.

I got her a new brand (very similar in nutritional value) and she immediately attacked the food as if she has not eaten in months. (it was pellets and no treats mixed in)

I tried to mix in just a few with her regular feed but she just picked the new bits out and is completely ignoring the old food, trying not to starve by eating hay all the time.

I wanted to know what could go wrong if I just go for a switch. Maybe as she is still eating hay, trying to keep her on mostly hay and an increasing amount of pellets could be better? I am really not sure but I know she will not touch her old food.

Any information or advice is welcome.