If upstream is nixpkgs, I use nvd (Nvd: Simple Nix/NixOS version diff tool):
which is part of a nixos-rebuild build --upgrade | nix-output-monitor ; nvd $currentsystem $builtsystem with a prompt asking me if I want to upgrade.
If I don’t, channel is rollbacked.
I guess that reusing the appstream metadata stuff for Software Center, it should be possible to build a true command that will ask upstream in reasonable scenarios, e.g. proper upstream metadata.