I took my angora's and new zealand whites off of pellets -
I transitioned slowly, over 6 months.
I was making my own chicken feed and mostly fed the same to the rabbits, with added veggies, herbs, maple leaves, raspberry leaves and varied grasses/hay. Whatever I had growing at the time. I would allow them to free range and watch their choices for new ideas.
The bird feed included Black oil SS, split peas, oats, assorted whole grains,
flax seed and kelp or sea salt. I added protein for the birds that I didn't add for the vegetarian rabbits.
It really isn't that hard if you vary their choices, include an 'oil' seed and mineral source.
We never had a rabbit die or become ill - and those who harvested angora always snatched up my rabbit's whenever I was willing to sell.
I hope this helps