You could remove the column from database, for example, for field 'margin' at sale.order model you could execute next SQL with psql:

your_database=> alter table sale_order drop column if exists margin;

Then update the addon in Configuration > Local modules

If you look at Odoo log file you will see one line like:

openerp.models: Storing computed values of sale.order fields margin

Export the model records, then import them.

This will trigger the write and the values for the functional field will be computed and stored.