In real life the pilot does not know the approach and landing runway when he is about to take off, correct?  that can change in flight due to weather and traffic, so on.  what do we do if we need to change the STAR after it is initially selected?  Can the flight plan be erased and reentered?  I tried to do this and found the process awkward and not easy.  It must happen that you want to land in airport A but due to bad weather you have to landing at airport B.  How does that change happen?  Is this advanced FMC programming or an easy change?  thanks. ::)