upgrade from 17.3 save pkg and repository info

I have to upgrade from 17.3 as I need the systemctl to install Virtual Shield. I used to be able to save all my installed packages using dpkg redirected to a file, backup the file and then install on the new system using dpkg and the back up list. I can't see that the install from the list is still possible - of course I might be remembering wrongly. The backup tool doesn't seem to work. On one PC I get the list but it is not saved to the chosen directory. On the other PC it doesn't find and files - both have almost the identical programs added. I've also noticed when I've packed up previously that the backup tool didn't list all my installed programs, which dpkg-l did.

I would also like to save my PPA sources to be able to add them again. I've copied the files from the etc/apt directory so I can add all by hand. I've installed Aptik but can't install the GUI - package not located. I can't find Aptik in the Menu either. The repository has been added and update run.