[otrs] Update from 1.1.3 to 1.2.3: Themes issues

Hello, I want to update our OTRS to the newest version and have already read the CHANGES and some posts on the mailinglist. Some mentioned that the old themes are not compatible with the new version. As we use a selfmade theme I would like to know what I´ve got to do to "convert" it. Cheers Lars