If you altered the refugee definition to remove the requirement that somebody be outside their country of origin, the practical problems of accessing them and trying to deliver protection would remain. I don't think that it's so much a question of whether you want to change the refugee definition. It's more a question of how you want to access internally displaced people and provide them with assistance and protection that is very difficult to provide.
Certainly many countries do permit various UN agencies to go in there and try to provide assistance to IDPs, but it's partial, it's incomplete, and it's highly contingent. It can change on a dime.