Depends how you have them currently configured.
If you are using Raid then you'll have to copy all the data off of that Raid, replace the HDDs and then copy all the data back.
If you are using SHR then you could replace the HDDs one at a time with a parity re-sync after each replacement. You will be without a parity disk during this process, so make sure your data is backed up.
Definitely don’t go for 7200 rpm drives. They will just be hotter and louder for no good reason. HGST drives are the best ones right now, but fairly expensive. I have 8TB Reds that I got from shucking external WD drives.
if you could borrow an external 8TB this is what I would do:
- Back up the whole NAS to the external 8T
- Power off
- Number all the old drives and remove them. Store them safely; they are your emergency fallback
- Insert the new 8T drives and setup the NAS as brand new
- Restore the backup
if it all goes pear shaped you still have the original drives which you should be able to put back in and restore to the original configuration (the OS and the configuration is all stored on the drives).