I can now answer my own questions, so I thought I might as well do this for future reference!
Yes, it's that simple. Changing back to the overview page is just as easy (tick the box again) and you don't lose any of your customization.
External users can have different levels of what they can see on the intranet. If they can only access individual groups, this shouldn't affect them. If, however, they have wider access of your intranet, they will also see the changes but won't be affected in a negative way.
People were confused as to why the 'Home' button was gone from the tab at the top of the screen. The 'Content' tab was now in its place. I think it might be a good idea to make a note of that beforehand.
Important thing to keep in mind:
Keep the community updated about the upcoming change (post a blog a few days before the change takes place) and make sure you can support the people that have questions about it.
These are my thoughts on this.
All the best,