In between r80.10 and r80.20, we replaced the upgrade code to be much more robust (its called the surprising name “new upgrade”).
By r81x we no longer supported the old code as the percentage of r80.10 management users was very small by then, and the focus on “new upgrade” enabled us to have fast and very reliable upgrade for r81x.
So in short it was done in order to have excellent experience for ~95% of the users when migrating to r81x. The cost is that the minority that left, have to go thru r80.40.
The recommended process is what is described above - we recommend going thru r80.40 and not thru r80.20.
Last but not least, we strongly recommend management of r81.10 over r81 (r81.10 has improvements in performance and many goodies). Also its good opportunity to update that by now, r81.10 has the highest usage when looking at management versions (among all the install base, more management users than r80.40 or any other version). The gw side usage of r81.10 also usage very fast but still r80.40 have more gw’s than r81.10.