To me, it makes no sense to swap Bamba for RWilliams mid season. Williams has finally gotten to the point where he can provide reliable minutes off the bench as the back up center. Bamba would be a step back for the rest of the season. Maybe some time in the future he is a slightly better back up center than Williams but I don't see the point of even thinking about this at this time.
And what we need is more of a PF player, not a center. Although still not great, our center rotations/options are still better than our PF options. You could argue that our best Center rotation would be Thompson, Theis, RWilliams. Out of necessity, our PF rotation is Theis, GWilliams, Ojeleye. If we were to find a way to get an actual starting level PF (and yes, those do still exist in the NBA), Theis could be used to back up both Center and PF, and both rotations get an upgrade. Bamba does not upgrade either rotation.