No, it's not the Welsh name that has changed, but the English.  For years Pen yr Ole Wen has been translated as "Head of the White Light", though no-one could explain what, or where, the light was.  Matters came to a head during a lecture on the Welsh mountains that I gave at University College Chester when Bill Hughes said that his father had always said "Light" should be "Ravine".  He pointed out that Wen is a mutation of Gwen, meaning white, and is the feminine adjective, so Ole can't come from Golau (light) because that is a masculine noun.

With me so far?  The matter has now been raised with Professor Owen at the Place-Name Research Centre at the University of Bangor.  He tells us that Ole in fact comes from Goleddf, which means slope or hillside.  So there it is, Pen yr Ole Wen means Head of the White Slope