Probably this solution is too late for you. However this is my solution:

DON'T ADD attachment=True to the model field. Here is steps you should do:

1) Copy all the files somewhere in order to save them. You could use XML-RPC script. If you don't have any files and you've got empty database you could skip this step.

• rpc_api.py (my RPC scripts helper api). You will need this file in step 5.
  

import xmlrpc.client as xmlrpclib

class API():
    def __init__(self, srv, db, user, pwd):
        common = xmlrpclib.ServerProxy(
            '%s/xmlrpc/2/common' % srv)
        self.api = xmlrpclib.ServerProxy(
            '%s/xmlrpc/2/object' % srv)
        self.uid = common.authenticate(db, user, pwd, {})
        self.pwd = pwd
        self.db = db
        self.model = ''

    def set_model(self, model):
        self.model = model

    def execute(self, method, arg_list, kwarg_dict=None):
        return self.api.execute_kw(
            self.db, self.uid, self.pwd, self.model,
            method, arg_list, kwarg_dict or {})

    def get(self, id=None, field='id'):
        domain = [('id', '=', id), (field, '!=', False)] if id else []
        return self.execute('search_read', [domain, [field]])

    def get_fields(self, id=None, fields=[]):
        domain = [('id', '=', id)] if id else []
        return self.execute('search_read', [domain, fields])

    def set(self, id=None, vals={}):
        if vals:
            self.execute('write', [[id], vals])
        return id

• Attachments script
  

from rpc_api import API
import os.path

if __name__ == '__main__':
    srv, db = 'http://domainname.com', 'database_name'
    user, pwd = 'admin', 'admin'
    api = API(srv, db, user, pwd)

    # Defining models
    models = {
        'model_name': [
            'field1',
            'field2'
        ]
    }

    for model, fields in models.items():
        # Setting model
        api.set_model(model)
        # Fetching all
        ids = [id.get('id') for id in api.get()]

        for id in ids:
            for field in fields:
                filename = "attachments/%s-%s-%s" % (model, id, field)
                # Check if file exists
                if not os.path.exists(filename):
                    for val in api.get(id, field):
                        base64 = val.get(field)
                        if base64:
                            # Writing to file
                            with open(filename, 'w') as infile:
                                infile.write(base64)
                                print("%s CREATED" % filename)
                        else:
                            print("model: \"%s\" id: %s column: \"%s\" is EMPTY" % (model, id, field))
                else:
                    print("%s ALREADY EXISTS" % filename)

2) Add attachment=True to your field in model.

fieldname = fields.Binary(string="Field 1", attachment=True)

3) Update/Upgrade your application and Restart server

odoo-bin --database=database_name --update=module_name

4) Delete field from the database. Execute following SQL script:

ALTER database DROP COLUMN fieldname;

P.S. if you don't delete column, it will save all files both in filestore and in database

5) Upload saved files (attachments) with following RPC script.

from rpc_api import API
import os

if __name__ == '__main__':
    srv, db = 'http://domainname.com', 'database_name'
    user, pwd = 'admin', 'admin'
    api = API(srv, db, user, pwd)

    path = 'attachments/'
    for filename in os.listdir(path):
        info = filename.split('-')
        if len(info) != 3:
            # Cannot be parsed
            print("Filename: %s cannot be parsed" % filename)
            continue

        model, id, field = filename.split('-')
        api.set_model(model)
        base64 = open(path + filename).read()
        # Setting value to record
        api.set(int(id), {field: base64})
        print("model: %s id: %s field: %s OK" % (model, id, field))

I don't know exactly is there any other proper ways to do that. This will work. Be careful with your database