The saying 'a change is as good as a rest' is a proverb; not a very old one. First recorded about 100 years ago. It means what it says: that having a change of activity is as good as having a rest. Thought to be of British origin, specifically the middle-class, one would expect to be called 'Dearie' while being told or reminded that a change is as good as a rest. "Well, you know, dearie, a change..."
Baceseras. (2008). [online] Available at: http://www.phrases.org.uk/bulletin_board/59/messages/250.htm (Accessed on 20/4/2011)