We have migrated the data from smaller pods to larger pods, then re-enabled backups on the new half filled pod to fill it to the brim. But we did not do this because the pod was necessarily old, we do this if a pod is unstable for some reason (usually it is a brand of drive we ended up not liking).
We have done exactly what you mention -> migrated off a pod and then disassembling the pod and using the older drives as replacements. Hard drives often come with a 3 year warranty, so for the first 3 years it is free to get replacement from the manufacturer if they fail. But after 3 years we have to pay out of our own pocket to replace the drives which can change the cost/math a little.