Hi abou,

If you want to migrate the database(s) yourself you can use OpenUpgrade, which you can find here: https://github.com/OCA/OpenUpgrade there is no support for a migration to V9 yet, though. You could upgrade your database from V7 to V8 but there is no script yet for V8 to V9.
Another option is to buy the upgrade through Odoo. You can find all the information here: https://upgrade.odoo.com/database/upload do know that this service is not free! At the moment the only good way to get to V9 seems to be by buying the upgrade from Odoo itself.

The custom code will have consequences in both cases, either if you upgrade the database or if Odoo does. You will have to get the custom code migrated to V9 too, which will cost you extra money. I'd suggest you to contact sales@odoo.com about this, since there is very little information about this.

Yenthe