We performed an import of all their previous bug reports using a new bug importer I wrote. This was the third Launchpad bug importer I’d written (the previous ones being for the Ubuntu Bugzilla import and a SourceForge importer), so I wanted this one to be the last. So the design of this importer was to have a simple XML format as an intermediate step. That way we only need to target the XML format to support a new bug tracker. This will also make it possible for projects to provide their bug data in a form we can consume for the cases where they want to migrate their bugs to Launchpad but Canonical doesn’t have the resources to do the migration.
For the SchoolTool migration, Jean-François Roche put together a simple Roundup exporter.
We should have some documentation about the intermediate XML format available at some point for projects interested in moving to Launchpad.