So, I tried the new update. Did not fail on PHP issues. Good.
But.... Following the log develop on the screen as the upgrade happened, quite a lot of errors roll across the screen,
various failures to upgrade databases, plesk, horde, ..
[registration error] Parent factory 'i18n-en' does not registred:
Unable to upgrade horde database from 2.x to version 3.0, see psa log for details
Bunch of failures to connect to MYSQL,
tr: broken pipe messages
Error : You can't use locks with log tables.
[registration error] Factory 'plugin-pp-op-bbva' share dir does not exists
Cannot create new FI due to 'admin' is already exists. (HTTP code: 404)
db-update-plesk10.php and Horde, each several times claiming I should rerun the upgrade preceeded by a whole bunch of "PHP Notice: Undefined index:" errors
Throughout the autoinstaller log it shows six times to "Please rerun /usr/local/psa/admin/plib/scripts/db-update-plesk10.php manually"..
Despite all the errors flying by on various topics, it ends the upgrade with a simple Continue button screen, claiming that Plesk components have been upgraded successfully, looking quite normal. And from initial login to Plesk, it seems to be running. But it is quite worrisome to see all the errors and claims that my detabases did not upgrade, despite that final claim to have been successful.
On the way, it also reinstalled some package(s) I had specifically de-installed because I don't use or want them, such as plesk-billing.
Probably belongs to the story, that my Plesk runs in Poweruser mode. (After originally having been in normal Hosting mode)
BTW. It flew into that final "Great Ugrade Success" screen without the usual chance to save the upgrade log that I remember seeing earlier. I did however go save all log files left behind in /tmp.