In the case of drive failure, the RAID rebuild will probably take days, not hours. If failure isn't something you're concerned with, or you only ever read and never write, it's not an issue.
Otherwise, you will probably be interested in replacing them sooner rather than later.
I would have expected that for a 2-drive system, a RAID rebuild would only be writing to the replacement drive. What data would be written to the old one?
Under that assumption, my strategy is to isolate these drives in their own 2-drive RAID volume until they fail.
Does anybody know if that's going to be a problem? Or am I fine as long as the next drive I add is CMR?