How to "upgrade" an image based on QCOW2?

I have a few VM images based on a minimal QCOW2 image (say release 15.2). Each VM's configuration is stored in a persistent image. Now, when there is a newer QCOW2 image (say release 15.3), how would I "upgrade" the OS to be based on the new image? That is I want to image have everything that the new (15.3) image has, except where I did configuration changes (like stuff in /etc).

I tried a qemu-img rebase, but the OS in the persistent image is still the old one (e.g.: /etc/os-release). So I wonder what the correct procedure should look like.

Also I want to get rid of the older "15.2" images eventually.