[AtlUmpSteve]

"A". It become, in effect, a 6' running lane.

As you note, the runner "can", but not "shall" or "must" run in fair territory. The runner is still protected if running in the original running lane, and if she elects to run in the 3' fair territory lane.

As best I can recall, the discussion regarding the interpretation centered on when exactly the runner might know if the defense elected the colored portion, and at what point the runner could or should be mandated to change. The result was to maintain protection for a runner staying in the traditional running lane, and to extend protection when the swap was made by both.