There's nothing in canon to address this.
I would assume when the host is assimilated, the symbiont would be assimilated with him or her. Nanoprobes could enter the symbiont through its attachment with the host. One could only speculate whether or not the symbiont would be given prosthetics of any kind. When the host eventually dies, there would be no reason to transfer the symbiont to a new host because all of its experiences would have already been assimilated by the collective.