Design pattern to enable replacement of underlying database at later date, with minimal efforts

The ones that pop into my mind first are the DataMapper and ActiveRecord patterns. I would recommend finding an implementation in your language of choice for whatever abstraction layer you choose. If you're considering writing your own version, take into account the time to implement vs. picking an off the shelf solution

For entity mapping you need a class that maps your entities to the database in the most simple way, using a Bridge or Proxy. You must also implement an Abstract Factory to handle database initialization, connection creation and all that stuff.