Slowly, little bits at a time and with good roughage~lower nutrient, stemmy hay and woody browse~available as well...it will help them balance out that rich feed. Any rich grass like that can cause some digestive changes and a little soft stool and that's true for ruminants as well. Rabbits are pseudo ruminants and they can experience some of the same symptoms.
So much so that for the best months of the summer they had to take very little grain based feeds...I was down to 1 1/2 c. of fermented feed for 14 LF birds and they were staying too fat even on that. When I processed a few I had the fattest carcasses of any chickens I've ever seen...and I've processed quite a few chickens in the past 37 yrs. I was amazed at how much fat this good pasture could put on a bird!
I have been seeing a lot about this topic and a lot of good reviews on a book called "Beyond the pellet". If you google "rise and shine rabbitry" you should have no problem finding it. Hope that helps.