Is it okay to switch from a back slab to a knee immobiliser?

Check with your doc: Without knowing the reason for the splint, the question cannot be answered. But more importantly, the "slab" was applied for a reason and you should check with your treating physician about changing from the "slab" to an immobilizer.

Don't know: You will have to give more information if you are to get a good answer. Why was the knee being immobilized? How long ago was it injured? Did you have surgery? If so, when? If it was a fracture, has it healed? Was it immobilized for comfort or for treatment?