In your Help menu, as well as in the Documentation folder, there is a file called "AC 11 Getting Started".
Starting on page 21 is a chapter on "Migrating ArchiCAD 9 Projects to ArchiCAD 11". Not that much work, but you'll want to follow the instructions there.
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For more information if you run into problems after that, see the Wiki. All of the migration articles were written for AC 10, but it is the same process:
http://www.archicadwiki.com/?action=fullsearch&context=180&value=migration&titlesearch=Titles
Karl
One of the forum moderators
AC 28 USA and earlier • macOS Sonoma 14.7.1, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB