I might be missing your question, but what is the difference?  Assuming your next waypoint has been set through an optimisation process, or is a corner of the course, how does a change over time alter that point?  If there has been a significant change in weather, re-run the optimisation altering whatever of your weather inputs are now different than the grib (e.g. Settings/Optimal Advanced/Wind).  And I may be missing the point of your question.