In the first screen of the Generate WebHelp wizard, select the default topic. You'll find it under content.
This is precisely what I did. I also renamed the project itself, not just the start page, but even after generating the primary layout all the RoboHelp system files (.glo, .hhc, .hhk, .lng, .ppf, .stp, .syn,...) in the !SSL!\WebHelp_Pro folder still depict the old project name.
Also, there are an "OLD_DEFAULT_TOPIC.htm" and further to this an "OLD_DEFAULT_TOPIC_csh.htm" and "OLD_DEFAULT_TOPIC_hha.hhk" files in this folder too, all of which gives me the feeling that my renaming of the project and creation of a new default topic have not changed the project's naming and initial setup in any way and that the new default file first needs to find the old one before it can open.
I'd like to A) change the entire project naming, so that the system files state the correct new project name and B) ensure that the new default topic opens without this "detour" via the obsolete old default topic.
Sorry that I misinterpreted your question. Exactly how did you rename your project? If you rename it trough the Project settings, you only rename the title of the project, not the files themselves.
To rename your project, choose File | Rename project. This will rename all your project files, if you use source control, check out all your files first!
To avoid the 'detour', don't output the WebHelp through the project toolbar. Instead, choose File | Generate primary layout and in 'select output folder and file name' set the folder you want to publish AND rename the filename OLD_DEFAULT_TOPIC.htm to the filename you want.
You can set the name of this opening topic to anything you want, the first topic is merely the topic that the help system (==OLD_DEFAULT_TOPIC.htm) opens.