It is not possible to open a current ActivePresenter project with a lower version.
ActivePresenter just allows you to open the current project by the current or later version.
In other words, the ActivePresenter 9 project cannot be opened by version 8, unfortunately.
There is no way to do that, Nico.
You can decide to create the project from scratch with ActivePresenter 8 or upgrade the license to ActivePresenter 9.