Fix it!
Look at the directory. Go to Folder Option -> View. Click "Show hidden files, folders, and drives" and uncheck "Hide protected operating system files (Recommended)". Click OK.
Now go to that directory.
Actually, the .caproj file is an .xml file. Use Notepad (or other text editor program like Visual Studio Code) to edit it.
Change the <saved-with-version> property to your wanted version.
One thing to see. You can see used plugins and more like your configurations, project settings, files,.. in your project.
Now, go back to the source. If you saved your project as a .capx file, extract it using WinRAR, 7zip,... Edit the .caproj again and open it. You've fixed!