I think the thought of moving Roy Williams to linebacker sort of slaps the face of good linebackers in this league.
Roy Williams rarely tackles anybody. He sites himself on someone from a distance and launches to make a big hit (never tackles). Working in the front 7, which is not his game, is a completely new learning curve. It requires him to actually read a play as it develops, in the middle of a ton of traffic. I just don't see that working. Linebackers are forced to read the development of a play and make key "tackles". Roy Williams is not known for either of these abilities.
Remember, Dallas actually worked this guy in preseason for the past couple years in linebacking roles, and it just didn't work out.