1. Open a bug report in a project for which you have permission to set bugs' importance.
2. Set its importance to "Unknown", and choose "Save Changes".
What happens: The bug's importance is set to "Unknown".
What should happen: "Unknown" shouldn't even be an option, because it exists only for bug watches, and the bug you're dealing with is in a project that uses Launchpad for bugtracking.
Suggested data migration: All bug reports that currently have "Unknown" importance, in projects that use Launchpad for bugtracking, should be changed to "Undecided".
Similarly, a bug's importance should be changed from "Unknown" to "Undecided" *whenever* the project it is filed against switches from not using Launchpad for bug tracking, or whenever it is retargeted from a project that doesn't use Launchpad for bugtracking to one that does. This particular change is not part of this bug report, though. It should be dealt with separately.
If, as part of the fix for this bug, "Unknown" bugs are not changed to "Undecided" whenever a project switches to using Launchpad for bug tracking, then Launchpad could still contain bug reports with "Unknown" importance filed against projects that use Launchpad for bug tracking. Therefore, when that automatic conversion *was* implemented, the data migration would need to be done again to ensure we'd caught all the errant bug reports.
The data migration could be done just once, at that later stage, but that would leave bug reports with the wrong Importance value between now and then. It might also cause awkwardness with the Importance menu having to include "Unknown" as an option only if that was the existing importance level, and not otherwise.