Most of the time key signatures were cancelled whenever the new key signature had fewer sharps or flats than the old key signature, in modern popular music, this is only done when C major or A minor replace another key.
↑Matthew Nicholl & Richard Grudzinski, Music Notation: Preparing Scores and Parts, ed. Jonathan Feist. Boston: Berklee Press (2007): 56. ISBN978-0-87639-074-0. "In popular and commercial music, the old key signature is cancelled only if the new key is C major or A minor."