Next it found out that there was not enough free disc space on the server. This is an incorrect message as I found out by brousing the FAQ and documentation files, so I had to find out how to disable the check. Another half hour later and I had removed the check from \lib\setup\ession.php.
So finally the new moodle started up.
But it looked horrible - text lines were overlapping and almost impossible to read. I traced this down to the default 'theme'.
I can not immediately transfer the mes 'theme' to the new version, but found another standard theme that looks about right.
Finally I uploaded the book module that we use to desplay the printable course notes. And I was able to open up the site to general users again.