I'm afraid we're not able to retrospectively apply this to existing routes - it will only take effect for new routes plotted. However, you can fairly quickly delete any existing directions you don't want by editing your route in the route planner.
Do we need to do anything to reduce superfluous directions from maps we have already plotted before this great change was implemented?
Following feedback on the plotaroute.com forums we've made some changes to prevent the directions that are generated when you plot a route becoming clogged up with superflous instructions like "Continue on...". This should help when it comes to navigating routes and should also help to keep the number of directions under the limit imposed by some GPS devices, as well as making the list of directions generally more manageable.
If you don't need directions to be generated at all when you plot a route, you can turn off the Generate Directions setting on your My Settings page.