It seems to me like the migrators is only used if they have a version specified. Ie. I have at site that used Our.Umbraco.Switcher, but I have to add [SyncMigratorVersion(7,8)] to it,to make uSync use the migrator when converting the original site. I also tried adding the migrator in the list of preferred migrators in the migrationplan, but without luck. Only when I added the attribute, it got picked up.

Hi, they should all default to 7 (only) if no attribute is specified