Back in August this site reported that the Transform 66 project, which (among other things) will widen 4 miles of I-66 eastbound from the Dulles Connector Road to Ballston, would not impact either the Custis or W&OD trails. It now appears that that was wrong. Istead it might make small, but real improvements to each.
On the Custis Trail, VDOT will need to rebuild the trail underpass near Bon Air Park and Westover. There the trail would be widened, and a sharp, 90 degree turn would be replaced with curved turn. It also appears that just east of there, the trail is modified a little to avoid something (a street light?).
Of course there would now be a 90 degree turn to the stub trail south of I-66, but I suspect that gets less traffic.
The W&OD Trail, meanwhile, would get a new trail bridge at Lee Highway to simplify traffic control at the intersection and eliminate trail conflicts. It would include a stub trail from the base of the overpass to the Fairfax Drive/Lee Highway intersection.
Construction of an additional eastbound lane is expected to start in 2018 and open to traffic in early 2020. I would expect these changes to be on a similar timeline.