How to upgrade and migrate data to new nas drives? 8tb to 14tb

Have a 2 bay synology nas (ds224+)with 2, 8tb hdds in basic configuration as independent storage pools.I have a total of 7tb data. I want to swap these for new 14tb HDDs I purchased and set up in the same configuration. Volume 1 is data I can't lose, volume 2 is only for media server so I'm less concerned about losing data. I do weekly back ups of important data to an external hdd using synology hyper backup. What is the best practice for swapping each 8tb drive with a 14tb drive and migrating the data as it currently exists?

Ideally I'd repourpose one of the 8tb drives as a snapshot of all current data (vol1&vol2) and use the second 8tb for continual backups of sensitive data.

Any recommendations or best practice for preparing, migrating, and after?