overwrite WRITE method and change the value passed through it as UPPERCASE conversion.

from openerp import models, api
class Product(models.Model):
    _inherit = 'product.product'

    @api.model
    def create(self, vals):
        if vals.get('default_code', False):
            vals['default_code'] = vals.get('default_code', '').upper()
        return super(Product, self).create(vals)

    @api.multi
    def write(self, vals):
        if vals.get('default_code', False):
            vals['default_code'] = vals.get('default_code', '').upper()
        return super(Product, self).write(vals)

update product_product set default_code=UPPER(default_code)