Awright! I've had my SSA3021X-Plus since Christmas, and I finally took the occasion to make some "tweaks" to it. The results are fan-dang-tastic!
My experience with the upgrade was pretty much textbook, following the direct flash advocated by tv84. The ONLY thing that I can add to the comments of those who have gone before is: Make sure you use good ethernet cables on your Telnet links. I had finished my manual backups of the original unit and was preparing to begin the reflash process. I plugged my USB stick into the back of the computer, right next to the ethernet port. Then the Telnet quit. Couldn't get it to work for anything.
Come to find out, the locking tab on the ethernet cable was hanging on by a thread, and the connector had worked its way out just enough to lose the link. Junk box cable for the fail.
I know I still have to get some good calibration standards to use the VNA properly, so that will be my next acquisition. I did break out the cal standards from my NanoVNA to at least establish an initial cal, and they worked sufficiently to prove that everything was functional.
Many thanks to all you guys on this forum topic that have contributed so many hours, sweat, and tears to make this possible. Tv84, electr_peter, Bicurico, tautech, and so many more... I have learned so much!
Now let the exploring begin!