Starting in R2018a, the apps you save in App Designer have a new format. This new file format might impact your ability to edit newer apps in previous releases, but it has no impact on your ability to run them in previous releases.
If you try to edit an app, created in R2018a or later, in an earlier release of App Designer, the new format is not recognized after saving your changes. You see a message such as this.
To enable editing of newer apps in a previous release, save the app in the release-specific format. Select Save > Save Copy As from any of the tabs in the toolstrip.
In the Save Copy As window, select a type from the Save as Type drop-down list.
The Save Copy As and Save As options serve different purposes, and their behavior is also different.
To save your app in a format that can be edited in earlier releases, use Save Copy As. When you use this option, App Designer saves the copy of the app in the specified folder, but it does not replace the app in your current session.
To save a copy of your app that is editable only with the current release, use Save As. When you use this option, App Designer saves the copy of the app in the specified folder and replaces the app in your current session.
If you open an app for editing that was created in a previous release, App Designer updates the app, and displays a message such as this one.