Update # 1
Last action yesterday was installing new version of WordPress, had a little test and found no issues. Now, I just set the logicals for all blogs to this version, so all blogs run using WordPress 4.7.2.
Update # 2
Following the instructions to upgrade MySQL 5.1 to MariaDb 5.5 didn’t work as planned. Partly because I forgot a few things, like stopping the running database (the scripts copies the database files, and they should not be locked…), but I also noted a number of options need to be commented out before upgrade – the script will fail because of that: It will take the 5.1 configuration and remove all comments, but doesn’t check on obsolete or now invalid options. No problem because that part of the update can be repeated. So in the end I ended up with a running server, but because I had forgotten to remove not just all files in [.data] but left the ones in [.log] alone, there were a number of issues. Resulting a very slow-acting database engine (had to restore the log files, amongst other things), but in the end ran into a stack overflow. Also found that creating a certificate for SSL connections refers to SSLROOT – which doesn’t exist – should have used SSL$ROOT, propably. So there is some work pending in this area.
So in the end I restarted MySQL 5.1.
To be retried later this week.