Thanks for your response.
I wanted to regenerate my configuration files to get my users and groups created again and receive all the possible updates from the new ispconfig version. I have to use the following workarounds, it seems:
I'm going to recreate all the ispconfig managed groups and users manually on the new server.
I'm going to copy the configuration files. They will be updated only when the site is changed, so I'll have to change each site needs any updates. Maybe none of them, at least I hope.
It is cumbersome this way, but expected to work, indeed.