After the image has been saved, can I still edit text-boxes & shapes in Preview.app? If I add text boxes and shapes to an image using Preview.app, save that image, and then close the image window, the next time that I open the image in Preview, all additions are uneditable.

In other words, all edits are made permanent upon save.

Is there a way to retain the ability to modify markup elements in Preview.app?
If you convert the image to a PDF, and then edit the PDF, you can edit the additions in Preview.app, even after saving.

To save an image as a PDF, you can use:


File → Export As PDF...
File → Export...
File → Print → PDF → Save As PDF...