I've wondered this for a while and never found a satisfactory answer. Free buildings, of course, are free - no build time, no build cost, no maintenance. However, what happens if the city in question already has said free building? Specifically, the CN Tower is available MUCH later than the Broadcast Tower. When I finish my CN tower, do all the Broadcast Towers I've already built revert to their free, no-maintenance versions? Or would I have to refrain from building ANY Broadcast Towers until near-endgame in order to maximize my benefit from this particular wonder?
The same question applies to other "free building" effects - Great Library (Library), Hubble Space Telescope (Spaceship Factory), Legalism (Monument), completed Tradition culture track (Aqueduct)...