collective.jsonmigrator is a ready tool for you to migrate from old Plone sites (2.0, 2.1, 2.5) to new Plone 4.0 (or higher). Its based extensivly collective.transmogrifier and custom blueprints avaliable.
What makes it different from other migration tools is that is fully documented and proven to work on lots of migration projects.
Real beauty of it lays in ability to easily customize it and extend to support all your custom content types that you are using.
(Note that collective.jsonmigrator was previously named collective.blueprint.jsonmigrator).