Success! Another converted Xmas-SSA!
Thanks to all contributors, special thanks to TV84 and Electr_Peter!
Some small hick-ups during the process:
#1 Putty didn‘d work for me too, it established a FTP-connection on Port 21 to the SSA. I used instead Win10-TELNET without any issues.
#2 I followed step by step elctr_peter's guide and with my basic linux knowledge it was done in 40 min
#3 I also had to remount the USB-disk in Step V with
(mount -o remount,rw /usr/bin/siglent/usr/mass_storage/U-disk0)
#4 after finishing the whole procedure bandwidth and licensing didn't work, before doing some nonsense on the console I went again thru
this thread and found the advice of dirtcooker msg #1659 as described above too and voila! it's done!
(deleting again /usr/bin/siglent/firmdata0/nsp_data_b1 once more, then sync && reboot)
Hope this helps other "improvers" in future.
Merry Xmas to you all around the world!