Couldn't you bump up the package version, and then make any new packages that use the new feature require the new version.
Hmm, I see what you're saying.
Perhaps we need an incompatible flag in the APM, which if set, means you can't upgrade a package without also upgrading another package at the same time.
Then any packages using the older notifications would have to be upgraded before notifications could be upgraded.
Would that work?
