You will not harm the racquet by stringing it at 64 or 65lbs.
I believe the change was based on updated playtest and general guidelines on tension from the research by the Wilson Performance Racquet Division.
Going forward Wilson will be using the following guidelines, by headsize, regardless of frame.
OS (106+) = 55-65lbs.
MP (99-105) = 53-63lbs.
MS (98 -) = 50-60lbs.
Chris, TW.