The migration plugin is part of preservation and is the way in Rosetta to migrate bitstreams from an obsolete format to an uptodate format. It is a script plugin receiving a file and delivering the newly migrated file.
For purpose of transparency it would be necessary to add a report about what has changed. For example when migrating pdf to pdf/a it might happen, that a font was included or even replaced. Having a report would implement a well documented preservation workflow. Depending on the migration tool the report might be quite complex.

So it would be nice to add this report as sourceMD on file level as part of the migration plugin.