One of the great myths about feeding rabbits. :roll:
What could kill them is not the grass but the sudden change in diet. Rabbits need time to adjust to new foods. Not allowing an ample transition time can cause them great distress and even kill them. But it is not the grass, per se.
If you are starting with pellet-fed rabbits, continue to feed them pellets for the time being. Add a handful of hay and a few blades of grass or other safe plants once you are sure they are eating and drinking fine after the stress of the move. Rabbits are very subject to stress and stress is cumulative. So go slowly.
Grass is only one component of a natural diet for rabbits. They need hay, preferably with a good alfalfa or clover content for the protein. They will need small quantities of grain as well - barley, oats and wheat are all good. And they will need a variety of fresh foods, of which grass is only one. Take a look at the sticky on safe plants at the top of the Natural Feeding section. Learn to identify with confidence the ones that grow in your area.
You don't say what your housing arrangements for your rabbits are. If you are planning on putting them on the ground in a colony, please understand that unless the colony is huge they will eat everything growing there in short order. So expect to be gathering greens for them daily. From April to October, I'm out there with my five gallon bucket... and I am feeding only a half dozen or so adult rabbits. It is work, but I like it most of the time.
This time of year, with the greens dwindling, is a difficult time to make a full transition to natural feeding. Although one relies more heavily on such things as pumpkins, root crops, grain grass grown indoors in tubs and vegetable trimmings from the kitchen to give the fresh component of their diet, it is more difficult than just going out an picking a bucketful of dandelions, chicory, plantain, clover, grasses etc. etc. You may want to keep your rabbits on pellets over the winter, adding hay to get their GI tracts accustomed to natural foods and then as the greens begin to appear in the spring, phase them in at that point. It will be a lot easier.
Before we started this forum last December, many of us were (and continue to be) members of the Homesteading Today Rabbit forum. There is a wealth of information we contributed still available there, mainly in the "stickies" at the top. I suggest you take a look because although much of it is here as well, we were at it over there for several years.
http://www.homesteadingtoday.com/forumd ... rune=&f=14