The grandfather clause was put in place NOT for the sake of people like George Washington, but for those of foreign birth who had helped in our Revolution. Men like James Wilson and Alexander Hamilton (who was born in the Caribbean, and who probably would've later become President except for the unfortunate fatal duel with Aaron Burr, Vice-President of the United States.)
So the "citizen" grandfathered in were those of foreign birth. Yet now those of foreign birth are "natural born citizens".
The link for your definition of the grandfather clause.
http://www.freerepublic.com/focus/news/3016232/posts?page=121#121