Hi,
I'm feeding mostly greens whenever available, grass (about 2/3) and all kind of safe weeds. It's what bunnys evolved for, it's perfectly safe and practical. Of course, I don't get the grow rates of pellet fed rabbits, but that's not my priority. Also, some big breeds might need some additional food.
I cant tell much about transitioning of rabbits that were on pellets/hay all their life, mine get greens from the start (except my buck which a bought with about 4-5 months, he was on hay/pellets/grains).
Another good point is that my rabbits have free run of the garden for hours, and I don't have to worry what they eat there, I removed most of the toxic plants, just to be sure.
There are some points to consider:
Transition: Do that very slowly, I would say over 2 weeks, starting with a few grass leaves now and then, and increase the amount day by day. Keep an eye on their poop, and always offer hay.
Bloat or diarrhea can happen when they get too much greens without having grown the right bacteria to digest it, and if not used to it they can eat too much at once, for most rabbits greens are mighty tasty.
Having baby bloat drops (Simethicone) at home is a good idea anyway when having rabbits.
What to feed: Everything that's safe, different stuff, and more than they can eat - once they got used to it they will pick out what they need, and spoil the rest :? (I have a BIG compost pile), not just dandelion alone because they like it sooo much.
Again, start slowly with new plants, the more diverse their food is the better. Some plants are easier to get used to, like grass, plaintain or leaves of rosea plants, others like clover should really be intruced slowly.
And, be critical about what you read on the internet, I got quite paranoid after reading too much about all those evil plants waiting out there to poison, torch and quarter my rabbits :runhills: , there are not that many plants that can kick a bunny out of its boots, if you're unsure about something, ask before feeding it.