The process can continue to change, new things can be learned, but the learning cannot be undone. If learning were to be a chemical change, the brain would no longer be a brain. It is not a physical change. In a physical change the materials are still present and we are not making anything new. Chewing is a physical change.
Digestion is chemical change.
Started with salt and water, ended with salt water. Made nothing new, just mixed them. Learning involves both physical and chemical changes. But let's not forget about the enzymes found in one's spit. Keep up the good work.