I was wondering whether making a currently public snap private has any consequences for current users of the snap. I understand that they will not be able to perform any updates, as they are not publicly available. But can these users keep on using the current version of the snap without explicitly being granted access to it when it has been made private?
Making a snap private in the store doesn’t affect the existing snaps that are installed on user systems. As you say, they won’t get any further upgrades, and if they remove it they won’t be able to reinstall it from the store, but they can keep using the installed version.