Pin-priorities are the solutionsolution; add this either to /etc/apt/preferences, or to a new file under /etc/apt/preferences.d:
Package: * Pin: release a=* Pin-Priority: 1001 apt upgrade will install the latest version of any installed package that it knows about, even if it means downgrading. apt only knows about packages from the currently-configured repositories, after apt update, so this will have the desired effect.