Just in case its not clear in the above instructions, the customization you have made inside the chroot environment will *not* be lost, even though you are selecting a new original *.iso file (ubuntu-18.04.2-desktop-amd64.iso) for the existing project. Instead, Cubic uses the file structure from the original *.iso to repair the file structure of your customized *.iso.
Just in case its not clear in the above instructions, the customization you have made inside the chroot environment will *not* be lost, even though you are selecting a new original *.iso file (ubuntu- 18.04.2- desktop- amd64.iso) for the existing project. Instead, Cubic uses the file structure from the original *.iso to repair the file structure of your customized *.iso.