I believe the German National Anthem was during the Nazi era.

The Soviet anthem had the words replaced, but kept the rather stirring melody.

S. Africa replaced "Die Stem van Suid-Afrika", but kept a verse of it in the new anthem.

Canada stopped using God Save the Queen.

Czechoslovakia's anthem was split (like everything else) right down the middle, but this is a weak example as it was originally two songs that were fused together (like everything else).