For all those needing to upgrade, I downloaded 1.10.3 from:
https://nxtforum.org/nrs-releases/nrs-v1-10-3/... but I'm still having problems as indicated above. 1.10.2 doesn't recognise the new 1.10.3 client and says it cannot install any upgrade until after the blockchain is fully downloaded. That's not happening because the blockchain is still stuck on block 10000012.
What am I doing wrong?!!! Do I have to download the entire blockchain again? How do I do that? Do I have to delete everything - all the downloaded previous client files and delete the NXT folder containing the database etc. and start all over again with 1.10.3?
You don't have to delete the old files, but if upgrading doesn't work for you - delete them, delete everything related to Nxt and install 1.10.3 from scratch. You'll be able to use it instantly in the Roaming Client mode while blockchain downloads in the background.