I was thinking of having a separate executable which would try to fetch a new version installed as part of the package, although maybe re-running the installer might be better concerning code duplication. It is definitely possible to have the installer provide an option to update a current installation.
Actually, thinking about it, it might be even better to turn the uninstaller into one of those modify/remove things and provide an option to update. This way you wouldn't have to keep the original installer, just the installed package. Seems pretty counter-intuitive though.