Assuming it can be done, which would likely require the following:

• quantum computing to model the neural pathways and configurations of every nerve cell in the brain

• the ability to program each cell to behave the same way as the one it will replace

• surgical precision at the molecular level to join cells properly

• the time available to surgically operate on 1 Trillion nerve cells

The most risky parts of the brain to replace would be the cells that make up the brain stem as these control basic functions such as breathing and heart regulation. Botched operations to other areas can leave the person brain-dead or lacking basic human abilities like speech and vision.

Assuming that the process is possible, perfect and can be done cell by cell… it would be plausible that the brain could be fully replaced if done in a strategic manner. However, we are far from a technological level where we can model the activity and behavior of every cell in the human brain. Also from a practical standpoint, if we can model and successfully emulate human consciousness… it would likely be first done by building synthetic humans.