I wondered this with things like FaceID. In my head, every time it unlocks it’s tweaking and tuning the what it knows to be the user, so it can adapt as a person ages, goes through weight changes, etc. However, in practice this may not be the case, since there is an easy backup and a person can re-register pretty easily.
On the topic of eyes, my dad recently had surgery on his eyes and they did one at a time, for obvious reasons. That could be a way to transition. Register both, have surgery on one, let it heal, register the healed eye, have surgery on the other, then register that. Always using the good and registered eye to authenticate. But this isn’t realistic. It requires way too much forethought and planning, when people’s minds are elsewhere.
On the topic of eyes, my dad recently had surgery on his eyes and they did one at a time, for obvious reasons. That could be a way to transition. Register both, have surgery on one, let it heal, register the healed eye, have surgery on the other, then register that. Always using the good and registered eye to authenticate. But this isn’t realistic. It requires way too much forethought and planning, when people’s minds are elsewhere.