Automatic update of field after save

Hello all,

Scenario:  A user creates an Opportunty, there's a "Region" field which is a Picklist with a few values.  They either make their selection or not.  They save the record and behind the scenes, an automatic function updates the record to match the Account "Region" field. 

1) Is there a way to do this using some kind of formula on the field?

2) If so, is there a way to overwrite what the user selected?

3) Idealy, the field is grayed out and the user doesn't have to make a selection.  The record gets updated after the save.

Any help or direction would be greatly appreciated.