I would go like a pinch of new pellets added into her old pellets. Like a teaspoon at a time added into her regular amount of pellets. Do that for a few days, then add another teaspoon for a few days. Then add another, 3 teaspoons = a tablespoon for a few days, then like half and half old and new, then a little less old, then a lot less old, then all new. It should take you like a week or two to finish out the transition.
Just make sure you keep an eye on her poop and make sure it stays uniform and not soft. If its getting soft, then you should give less new pellets. She'll probably be okay, but you should just try to transition her slowly, because she might be a little stressed out; since she just got to your house.
I had to transition my rabbit ON to pellets, but it still took me like a week or so to get her eating the pellets. She ate those for a few months and then I had to transition her again to another pellet because she wasn't doing well on the first ones. So that took like 2 weeks too, because the new pellets had more sugar in them and I didn't want to upset her too much.
How much pellet is she getting now? I think with her weight, she should be getting like 1/4 cup a day. What pellets is she eating and what are you switching her to? I'm just curious, I'm always interested in what and how much other people feed their buns.
Good luck transitioning!