The lookup page for schemas shows a writing schema 6 all the way back in the Gramps 2.01 (2005-05-23) through 2.04 (2005-06-27) versions.
After loading the old database, you would want to backup to XML, not incremental stepping up the version of the database. By exporting to XML, Any modern version of Gramps can read the data.
Gramps 2.2.10 (2006-02-27) was the last version that read database schema 6 and it wrote 1.1.4 XML schema.
If you were to go the route of incrementing the Database schema: Gramps 2.2.10 was the last version that could update that schema 6 to database schema 12. And Gramps 3.0.4 (2008-12-06) updates to schema 14. All Gramps versions after that can read database schema 13 or better.