Dieter,
One way to do it would be to use the export script. Check this [Upgrade Documentation page(https://gluu.org/docs/deployment/upgrading/).
In the next version of the Gluu Server 2.4.3 (due out any day... its in the final QA), we've externalized all customizations, so that will make it easier to use a tool like git to save your custom pages, scripts, libraries, etc.