Migrate Odoo 14 db to Odoo 15

Hi, we have a client who is currently using Odoo 14 but needs the service to manually migrate its db to Odoo 15. The client doesn't have access to the Odoo 14 custom modules, so the task is to manually dig into the Odoo 14 dump, arrange the data that is going to be migrated to Odoo 15, and import it to the new database. We have not yet decided which models will be migrate it yet but is likelihood that there are going to be: models related to account_move, sale_order, purchase_order and stock_picking.

The task is estimated to take some weeks or months, and we are looking for someone with strong PostgreSQL and Odoo Skills to be able to do this job with accuracy. Happy to hear proposal on how to carry on this task,