There was a discussion somewhere about 'Happy Birthday', written 1932, I think. The tune is based on an older tune, which is apparently out of copyright, but the actual modification (Two notes for the word Happy, where the original had only one) is still IN copyright, and has actively been protected.

If you are familiar with a song called 'Two little boys', check out 'Two Sailor Lads' (from 1906) in the Levy Sheet music collection, for just one example of possible recycling.