More specifically, if the computer HD is APFS and the backup/cloned drives are HFS+, why change the backups?
There's no need to change them with CCC 5, but a better question is why maintain any HFS+ volumes at all (other than for 10.12 and earlier installations) once you're satisfied with HS in APFS mode?
As I see it, an external HSF+ volume is no more than a short term stopgap measure for when you first upgrade to HS...until you're satisfied with APFS.
