As much as I love Hibernate I would not consider H to be the best tool for the job.
I would rather recommend using iBatis for that.
Or if you already have one H mapping to the old schema then you might better off creating iBatis mappings for the new schema and persisting Hibernate fetched objects from old schema to new DB schema via iBatis session. H-2-iBatis approach worked very well for me about a year ago.
Or maybe specialized tool like Octopus
http://octopus.objectweb.org/ would do the trick (did not try personally, but it looks promising).