The replacement for the 2040 is going to be 2520 or may be a 2554/2.
Given Netapp pricing wrt to new systems versus just shelves (a whole system can cheaper than a shelf with the same capacity) and likely maintenance costs for a very old 2040, a new system is by far going to be the better solution.
Assuming you want 400TB usable of SATA space, I'd be going for a 2554 with 24 internal 4TB disks and then 7 or 8 shelves of DS4246 with 4TB disks. If you need more performance make 10% of the disk SSD and use flashcache.
Although there is a new family of NetApps coming, the 26xx - might be worth looking at or leveraging to get a good price on 25xx.