Microsoft updated their Bluetooth drivers etc. so the block was removed & I could upgrade our last bit of hardware to 2004. Just a few quick notes in case they might help anyone else...
The tablet only has 32 GB of storage, and while 8+ GB were free, for the 1st time that wasn't enough, and setup asked for more storage. First try used the inserted micro SD card, and that didn't work -- setup couldn't find it when the tablet restarted. 2nd try I used a fast USB 3.0 stick, and while that worked, the update process itself was so slow, using the USB stick, that the battery ran out during the upgrade. Luckily after charging I restarted & it was able to resume setup, finishing the installation.