Slow replacement at the cellular level already happens now too.
Inside your cerebral spinal fluid, there are neurological precursor cells. Those cells HAVE been seen to integrate with the brain in damaged areas. Human brains damage themselves when under stress through poisoning by beta amyloid and other such things, like tau protein tangles. (There is still raging debate over weather these features cause the damage, or the damage causes the features though. There has been some animal model studies that show that a good night's sleep allows these products to be dissolved into the CSF, then whicked out of the brain before they can form plaques, which suggests that their presence is causal for those disorders, but this is still very preliminary.) dunno if any of you have univerisity-level access to scientific publications, but if you do..
http://www.ncbi.nlm.nih.gov/pubmed/22907556From the abstract:
Restoration of the damaged central nervous system is a vast challenge. However, there is a great need for research into this topic, due to the prevalence of central nervous system disorders and the devastating impact they have on people's lives. A number of strategies are being examined to achieve this goal, including cell replacement therapy, enhancement of endogenous plasticity and the recruitment of endogenous neurogenesis.
*emphasis mine.
If you are good and healthy, your brain *IS* slowly being replaced with complex replicators that assume the function of dead or sick tissues already.
You would not notice if your brain was slowly replaced by artificial ones either.
To whit-- most people dont notice
even when they have a brain riddled with swiss cheese holes from having exotic brain parasites in there. (yes, brainworms are a thing. Enjoy your undercooked pork!
This is why your GF insists the kitchen counter be spotlessly clean kids.) The usually reported symptom given by sufferers is mild confusion, and chronic headaches. Very similar to developing migraines. As the cysts enlarge, it causes hydrocephaly, and seizures. In cases where the number of cysts is low, these latter stages may never occur.