They are not going to put Lewis at RT. If it were even something they were considering they would have done it when the opportunity was there to do it in the season. Instead they did not and kept him at guard because they view him as a guard and since they see him practice I am more likely to believe them. Also it is not a "slight" downgrade, you are going from top 5 at their position to likely bottom 5 at their position which is a massive dropoff.
The Ravens entire offensive line woes stem from weakness in the interior, specifically that none of them were really healthy and couldnt really get any push in the run game. However the outsides were great especially as the season drew to a close and Stanley starting looking less like a rookie and more like a top LT in his prime.