mdadm RAID1: can you grow to 3 disks, then revert back to 2?

I have a Linux (CentOS7) software RAID1 system with two disks which are getting old. One has been starting to show signs of failure. I'd like to phase these disks out one at a time with new disks but always have a fully mirror active. I was thinking to add a third new disk for a 3-way mirror, then fail out an old disk, repeat with another new one, then fail out the second old disk. That way I'd always have a mirror and not wait for a raid rebuild after a fail event. This seems like it could work with num-devices=3 and grow, but, when I'm done how do I 'shrink' it back to two devices? I can't seem to find a reference to what I'm attempting.