How should a Traffic Control Tower notify aircraft that their frequency just changed?
ATC frequencies don't change very often, but when they do they are planned well in advance.  NOTAM's are issued ahead of time to let people know, and if the airport has ATIS they will include the new frequency on the ATIS.

If it is an enroute frequency, the other controllers will simply hand you off to the new frequency, and you don't even need to know about it ahead of time.