"Goodbye" by Mary Hopkin. The Paul McCartney composed and produced followup hit to "Those Were The Days" by Mary Hopkin. Hey, even that would be a good closing song.
At the station I'm at, since 1974-1975, when we finally went Stereo, everytime some major technical event happens at the station, the last song played has been "Those Were The Days" and the first song aired when we came back on, or made the switch, or whatever, has been "Ding Dong, Ding Dong" by George Harrison whose main lyrics are ... "Ring out the old, Ring in the new". The most recent time we did this was in 2001 for a power increase.