As far as I can see right now, export/import CMDB data is my only option.
No way you can update from 1.4.8 communitiy version to whatever.

Nope, it is not correctly done. The installation procedure needs to be documented in better ways cause you can't update from 1.4.8 to 1.9.2 unless you know the right steps (no where to be found)
Now I will assume I need to update to a next major release first, skip any minor releases and do major updates only until the last branch 1.9 is reached…

Or just some plain simple import/export of my CMDB items from 1.4.8 to a freshly installed 1.9 would be fine too.

I am not able to update 1.4.8 (Community version) to 1.9.2
I tried to update to 1.9 and even 1.8 but it still comes with the same PHP error that some class isys_auth_system_tools is not found.

Eventually I need to update and upgrade the entire installation from Windows Xamp and i-doit 1.4.8 to Debian 9 and i-doit 1.9.2

What can I do to make this work?

Update before migration?
Or migrate i-doit 1.4.8 Windows to Debian 9 first and then update? I guess the same error will come on Debian as it looks to me like a PHP script issue.

@15.20
At this moment I installed Debian 9, MariaDB10 and PHP5.6 and have migrated from Windows to Linux succesfully.
Downloaded idoit-open-1.9.2-update.zip and extracted the zip, uploaded the folder updates, replacing existing files.
And as expected it throws an error:

Fatal error: Class 'isys_auth_system_tools' not found in /var/www/html/cmdb.mydomain.local/public_html/updates/update.inc.php on line 137

@15.47
Never mind, found the proper way to install the update. It works… Just needed to copy new version on top of old replacing all files, not just the folder updates. (it's a new release...)
Now I can remove PHP5 and install PHP7.