Recommended Approach for upgrading drive

You cannot have two volumes of the same name nor is it easy to change a volume name (see Changing-the-volume-name-on-an-os6-based-nas if you really want to do it).  To get what you want, you'll need to use your backup to restore volume contents after you DESTROY and then replace each or an intermediary drive/volume.  Since using an intermediary drive entails two copy operations, NAS-to-NAS copy from the backup may not be a lot slower.

To use an intermediary drive:

Add Volume3 in the empty slot.

Copy contents of Volume2  to Volume 3.

DESTROY Volume2.

Add new Volume2.

Copy contents from Volume3 to Volume2.

Delete contents of Volume 3.

Copy contents of Volume`  to Volume 3.

DESTROY Volume`.

Add new Volume1.

Copy contents from Volume3 to Volume1.

You could use an external drive as intermediary instead, but it will take more time.  Restore from backup is probably at least as fast.

If the drive in slot 1 isn't the primary volume (contains home folders and apps), then there is more to do.  See How-to-save-your-apps-when-destroying-your-main-volume-OS6.   If that volume was the first drive you put in, it should be primary.  Otherwise, one of the others is.  This extra step is needed if you use home folders or have apps installed, regardless of the method used to copy the volume contents.

FYI, the SSD with the OS is probably doing less than you think.  The OS is in RAID1 on all drives, so having one with the OS only doesn't speed up OS access.  It will speed up home folder access and app access if it's the primary drive.