Remember, a 26" shaft is measured from the back of the point to the throat of the nock, not from end to end.
No, a 50# shaft cut to 28" is still a 50# shaft. However, the dynamic spine of the shaft will react as a 42# shaft cut to 26# would react, much the same as a 50# shaft cut to 26" will react if it has a 190gr point instead of 125gr point. Both length and point weight will affect dynamic spine, but the shaft will still remain a 50# spine.