Replacing patio stones with grass
Well chances are that below your stones is 5 - 8 inches of gravel. The amount of gravel depends on your climate and if the original patio job was done right. So to properly grow grass you will need to remove almost all that gravel and then fill it with soil. This would be lots of hard labor.

I would probably dig up a bit under one of the patio blocks and see how far down the gravel goes. If they used no gravel you could just remove the patio blocks, top off the soil a bit and plant the seeds. However, it is unlikely they didn't use gravel as your patio would become uneven fast.