Each major version of Designer stores its project files in a version-specific folder.

Whenever you open a 2.9 (whatever) older version project in 2.11, it will automatically convert it. You will then have to save it in the new 2.11 project version file.

I recommend keeping your 2.9 project files as-is (just in case) and making new copies of them for 2.11.

Your 2.9 project files are most likely in

C:\Users\Public\Documents\Do-more\Designer2_9\Projects

Your 2.11 project files will want to store its files in the 2.11 folder

C:\Users\Public\Documents\Do-more\Designer2_11\Projects

You can open the 2.9 folder in Windows Explorer and open 2.11 folder in a 2nd instance of Explorer.

Basically copy the contents of the 2.9 projects folder into the 2.11 projects folder.

When you run Designer 2.11, browse to the 2.11 projects folder and open those as-needed. Then just save them (no need to do save-as since these are already copies). Key point is to make sure you are opening the 2.11 projects folder .dmd files (not the ones in the 2.9 folder)!

Other people out there may have other ways on how they migrate their projects. Some care about keeping old version files around. Some don't care at all. Some care about specific project files, but not others. Some store their projects on a central server and tweak the default Projects folder setting in Designer to point there (see File->Manage Projects->Folder Settings). It's always interesting to see how other people utilize features.

EDIT: You can always open OLD projects in NEWer versions of Designer (the old project will be migrated to the new file version, so do a Save-As if you want to maintain the old file as-is).

However, you cannot do the opposite - open a NEWer project in a OLD version of Designer. You get this error dialog when attempting to open a 2.11 dmd project file in 2.10 Designer: