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?