andi Thank you for your answer. I read it and tried to understand everything.
To Q3: For my question 3 I have the obvious answer, that it is less complicated to do an upgrade. Thats a big advantage!
To Q4: I have the feeling that the splitting of my dokuwiki into
/var/www/html/my-wiki/ will make it complicated to use the plugin. I understand, that I can delete the data directory at the default location afterwards. But I don't get what is meant by
You may need to create any new subfolder inside your data directory manually after the upgrade.. In which cases I have to do this? And also I don't understand, what I then have to do.
To Q1: In the manual is no information, whether I should go from H to I first, before I go from I to J. As I think the plugin does not work for me (see above), I would go the step-by-step-path for the manual upgrading, described here: https://www.dokuwiki.org/install:upgrade
To Q2: If I do the manual upgrade, I am still unsure if I should click the Checkbox
Download changed files only at the download prompt. I have the fear if I check it, I have the fear that not all neccesary files are there, as I am upgrading from a very old version. If I don't check it, I have the fear, that it might override files, where I have settings or so stored to the default, crushing the system.
thank you very much in advance.