That makes zero sense. You dont need to upgrade first because you're reinstalling the software as a part of the conversion. upgrading it first to then wipe and zero the drives serves no point what so ever.
I just recently used the NetApp KB for a client DR site where we were re-purposing a FAS3220 from 7-Mode to cDOT.
The only difference between what I ended up having to do vs. the KB was that there were no existing roots on the FAS3220. The KB doesn't cover that part. The software installing process from the boot menu in the KB isn't a netboot image. If you don't have a root, you can't install software.
I found the YouTube link which has the slightly different process at the start using the netboot image. Just remember that you have to extract the netboot image first before pointing to it. That process worked like a charm.