Hi,

If you add a new function field to an existing module then there will be two possibilities.

1) Field is store in database : Odoo will automatically calculate value for the all existing records when module is updated and field is created. Odoo will call function for all existing records and calculate value for all of them and store it into database. If there is an error in the existing records according to the function then it will not calculate data and field will be blank for existing recoreds.

2) Field is not store in database : Odoo will automatically calculate value for the existing as well as new record as on when its value is required.

So, as I think there is no need to do manually calculation for the existing records.

I hope it will help you.