I dont think the issue here, is that netapp doesnt document where to download the PROPER version of ONTAP.
Go to the NOW site at http://now.netapp.com/NOW/main/software.shtml and download the version of Data ONTAP for your existing system that matches the version installed on the FAS31xx system.
But..ya think you're reading "upgrade version" a little too...narrowly?
How can Netapp prevent this chain of errors?