Migration of DB with custom modules is totally possible and supported by our migration platform. Here are the detailed steps:
1) The partner dumps his customer DB (the whole DB with custom modules)
2) He uploads this DB on our migration platform: http://migration.openerp.com/
3) OpenERP does the migration job:
The important fact is that we migrate the original database in place. All is done in the original database. For example, we restore the v6.0 database and execute all the scripts in this database. At the end of the process, this v6.0 database becomes a v6.1 database.
4) the partner will then receive an email with a link to his migration request status page where he can download the migrated database.