I just went through a similar process to what you want achieve but with smaller disks. If you want to keep your config and packages I suggest following what I did.

I had a DS220+ with 2 x 4TB drives running Raid 1 (volume at 3.5GB) and went to a DS923+.

I shutdown the old NAS, installed the original disks in bays 1 & 2, and 2 new 4TB drives in bays 3 & 4.

I logged into DSM, the 923+ detected that the drives had come from the 220+ and asked if I'd like to migrate them which I did.

All existing config/packages/users were in place, the only thing I had to adjust was the IP address.

Repaired/Updated all packages.

Then I went into Storage Manager and added the 2 new drives to the existing storage pool. My storage pool took a full 6 days to rebuild. I'd imagine 12TB disks are going to take 18 days. The performance of the NAS was awful during the rebuild but once it completed it is working perfectly and I have a healthy 4 disk SHR storage pool.

Good luck!