Hi Timo,
Yes, you’re correct. The safest route when making a major change in your site is to do it first in a Staging site. It’s also best practice to take a backup of your site as well.
After doing these, in your staging site, you can proceed with importing “App” from the Site Library. If you have another site template imported, you’ll need to remove it first, but take note that anything you added inside a resource/page/element that was imported through that template will also be removed. Reference: https://docs.generatepress.com/article/removing-imported-site/