A few years ago I got the Limited Edition M&M I-VI. Now I noticed that the early M&Ms are downloadable in single files and that there is a newer version of M&M 4/5. Before I break anything: Would I first need to uninstall the older Limited Edition or can i just install it over the older version?

You need to uninstall Limited Edition first to avoid tainting Windows registry. You can make back-ups of save files, they are in game directory, all files with extension "SAV".

I thought so. Thanks for the fast answer. Worked like a charm. ;-)