Hello diskono,
When you are using Change Path command and select Apply to all cameras option - you should specify the path to the single file of the list only - it should not be greyed out. However, make sure that the file names are exactly the same as before, since the change path command is case sensitive.
If the problem persists, please provide some screenshots (you can send them to support@agisoft.com, if you do not wish to make them public) demonstrating the camera labels in the Metashape window, files in the Finder as a list with the extensions specified and "change path" dialog.