Yeah, this is a pre-emptive replacement--the drive is not failing. We're trying to pull out the older drives before they do become a problem. Nothing tightens the old sphincter like a mirror with a failed disk we're replacing sitting side-by-side with a disk of equal age that we're doing a massive read operation on to re-establish the mirror.
The statement that disk a and disk b (in a healthy mirror) and we're replacing disk b with disk c, if something did happen during that replacement with disk c, disk b would still be a member of the mirror. Is that accurate? I don't like the idea that the process may offline disk b in order to replace with disk c and something happens to either disk a or disk c during the replacement without being able to fallback on disk b. Oh the paranoia... Now I'm probably overthinking things here.