Could humans see any of their organs replaced by their symbiotes? Of course, this already happens if folk are disabled, they can have artificial limbs, and organs. In the long run, all our organs will probably be replaced. The brain could be one of the first to go. We have already heavily outsourced memory. If prediction capabilities become outsourced, the human brain may have a much reduced role to play. Also the human gut may become redundant early on. If food comes predigested, a complicated internal food processing system is no longer needed.
Oh, and I should probably warn you, the first video is not very nice: contains death.
Cymothoa exigua - The most horrifying parasite.
Fish Parasite, Cymothoa exigua?