I think it was in Properties where I changed the Board type but I can’t be sure. There was a warning but I didn’t expect all my Projects to disappear. Perhaps a better option would for different board projects to be placed in different folders so that they are not deleted.
If you select a new board in the project settings, the basic settings of the selected board will be displayed. This is because if a board has a display with a specific resolution and color depth, it must be used in every image in the project.