Grass is okay to feed rabbits, however if your rabbit is not used to eating grass it could make them sick. You have to slowly get them used to eating grass like you would with any other vegetable. While at the same time if you just stopped feeding your rabbit hay that also could make them sick and upset their digestive system, so you’d also have to gradually reduce and get them used to it.
As far as a complete replacement for other hays I would check the nutritional value in grass (and find out exactly which type of grass you have) and compare it the nutritional values of other hay. There might be key vitamins, ect. that are in other hays. Also, I’m not sure if the texture makes a difference because hay is rough whereas even dried grass (you’d feed the grass fresh though) is not and the roughness is something they might need for their digestive system or their teeth.
Sorry if that was confusing! Hopefully someone who knows more about this will chime in 