Mash vs. pellets .vs crumbles is only a matter of texture, not content. It all starts as mash (basically the raw ground grains, minerals and other additives) and is then processed into pellets or smaller crumbles.
Apart from that, yes, it is time to switch them off to a layer feed of whichever consistency they (and you ) prefer. If they're just starting, it's not likely they're all starting at once. Start feeding layer cut (heavy on the layer side) with decreasing amounts with the last of your grower until you run out of grower and have them on straight layer. At the same time, make oyster shell available to them immediately. They'll all go at it at first out of curiosity because it's new in their world. Those who need it (they KNOW!) will take what of it they need.