I figured out that if you detach the repo prior to deleting the backup copy job (in step#5 above), it will retain the original retention dates. I plan to use this same process when I move my production data over in a few weeks. If the retention dates get messed up though, I'll just use the "Delete backups in bulk" functionality once a week to clean my repo storage. The new arrays have a much larger storage capacity, so leaving a few extraneous recovery points around for a few weeks shouldn't cause any issues.