In Bellevue, WA in 2008 a bridge across I-405 (at NE 8th) was constructed next to the old bridge, ANOTHER bridge constructed on the opposite side, the old bridge destroyed, and the first new bridge slid into place creating a new bridge twice as wide as the old original bridge.
Watch the video:
The bridge on the left used to be a two way bridge.
They could not construct the bridge right next to it since they would have had to close the original bridge (since it was so close to the new construction).
So the built it off to the side. Once it was built they opened it to traffic (temporarily) presumably to get a head start on converting the nearby roads to match the new traffic flow (which they would need to do anyway).
Then they slid it into place and the new traffic pattern is already set up. They actually could have left it off to the side but that wouldn't match the existing roads as well (too many turns).