You can definitely switch pellet brands, but I’d do it once she’s more comfortable with you (as to avoid any stomach trouble).
I’d add a ceramic tile (make sure edges aren’t sharp) so she can lay on top if she’s hot.
Edit: She might be a little hesitant to eat so I’d ask if she...
Take a picture of it off Bing
Identify what it is (tells you)
Look at Poisonous Plants | House Rabbit Society
if it’s toxic go to your vet
Also go to your vet if plant was sprayed with pest side or fertilizer
Well if a rabbit is in pain ( UTI, ect. ) they’ll typically be dribbling ( tiny puddles of pee ) on the outside of the litter box, which makes it easy to tell
Information: Red Urine: Blood or Plant Pigment?
I might add that outdoor rabbits are much easier to lose interest in & having them inside its easier to interact with them & see their true personality.
But regardless thank you for not just dumping them. 🙃