I can't find specific reason for renaming to Memorial day. Perhaps George can instill some history/memories upon this that seems to be forgotten (or ignored?).

Here is the best link I could find,
http://news.yahoo.com/memorial-day-7-historical-facts-holiday-143814167.html

All sources seem very vague as to why. Sure, they give a date as to when the day got moved. Perhaps they renamed it to make it clear that the new date(Memorial day) would not be confused with it's predecessor(Decoration day)which was a different date. Yet some sources seem to say that it was being called Memorial day before it was officially moved to now.