She should have unlimited hay, so you shouldnt worry about her going hungry

The old food evidently smells a lot better to her. Rabbits can be very discernible. When you do switch pellets, you’re supposed to mix 75% old with 25% new for the first week, 50/50 the second week and 25% old with 75% new the third week. I don’t know if that’s how you did it, or if it makes that much difference what the percentages are if the rabbit is going to rebel against the new anyway. I would try leaving those remaining new pellets in the bowl without adding any old ones back in, and see if she will eat them. If they are still there after a day, you should probably replace them, though.  You didn’t say how old she is.  Do you think you might be feeding her too big of a quantity of pellets?  Maybe if you cut down the quantity, and still tried the mix, she might eat all of them.  You could also try hand feeding her some of the new pellets, if you think that might make her  more eager to eat them.

I agree with Toby. Leave it in – she won’t starve with hay and veggies. That’s what I did with one of my stubborn rabbits who refused hay but loved pellets. Just left the hay there and until he ate it, no pellets. Now he loves hay and during that time was perfectly fine.

If she still won’t, mix very little new food in and progress from there.

Don’t want to be judgmental.  Our two Flemish girls primary food is green vegetables.  They eat carrot tops, dill tops, fennel, cilantro, parsely, dandelions , bok choi, romaine lettuce.   We don’t feed them all of this all at once.  We feed them several green items at one time, they eventually get tired of eating the same thing, and then we switch to new greens.  They do seem to always like a chomp of romaine lettuce.  Their follow up food is oat hay.  Oat hay has oat kernels, which they like.  Also, the oat hay is abrasive and does help wear down their ever growing teeth………They do have pellets. I try to buy pellets that don’t have sugar/molasses to prevent them from excessive calories. This is for them to munch on when they want to.  Treats…this is where they probably get some big calorie inputs.  They really like banana slices…they like tart banana’s with a little green skins.  Apple slices….they want Fuji apples.  Actually it’s the treats that makes them really friendly pets…the two Flemish sisters, given greens, hay, and pellets they could live their life by themselves, it’s mainly the treats that motives them to come out and interact with us. At a year old, they are starting to be more and more interactive with us, particular the shy rabbit sister.