Atom text editor https://atom.io is installed on Ubuntu 20.04 and used by single user. It was installed from Ubuntu classic binary package. The mission is for this machine to migrate Atom to snap app. For this all what was done so far is to install Atom snap. In next step migration should be completed. The goal is to retain UX - complete local and user-specific configuration.
However if to start Atom snap by executing snap run atom the running instance has access to all packages installed to Atom classic installation and few others (recent oopen projects, recent open files lists). Means Atom snap uses yet gets visible local and user-specific configuration generated once by Atom classic package. This way it is hard to see how to conduct migration steps.
The Atom snap is a classic confined application. As a result it has access to the same configuration / plugins and documents that the "classically" installed binary package would have. So there's not really any migration to be done. Simply remove the deb, install the snap, and you're done.