To open a MAXQDA 12 project file with MAXQDA 11 is fairly simple.
Start by opening your MAXQDA 12 project with MAXQDA 12.
Choose Project > Export MAXQDA exchange file in the menu to export your data via a MAXQDA exchange file (.mex).
Save the .mex file at a place of your choice (e.g. your desktop).
Open MAXQDA 11 for Windows (11.1.1 or higher) or MAXQDA 11 for Mac OS X (11.2.3 or higher).
Choose the option Other Projects from the welcome dialogue.
Select the file format MAXQDA exchange file (.mex), go to the saved file and click Open.
You should now have successfully opened a MAXQDA 12 project with MAXQDA 11.