In regards to the brain, how long does it take to rewire? would a brain scan be able to distinguish a difference, say from time a to time b?

Rewire ? The linkage between brain cells may/may not ever re-wire. The older you are the less chance recovery to any injury can occur. The idea of re-wiring depends on teaching the body to use accessory pathways to accomplish a task, not reviving those lost. PET scans would have more value in determining activity change. Regular scans likely have little value.