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

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. Would you like to video or text chat with me?

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? Would you like to video or text chat with me?