Are VOR frequencies static?
Charts are updated regularly by the FAA and expired charts should not be considered legal for navigation. Your chart must be current as per the date range printed on the cover for it to be considered reliable. That being said in my experience the VOR frequencies remain constant although I'm sure you can find instances in which they have changed over the years I have yet to see it happen. 

FWIW the FAA is slowly shutting down the VOR system as GPS has become the prevalent navigation method.