‘Ballad in Plain D’ uses the broadly same melody as ‘I once Loved a Girl’, and takes that first line, and twists a few later ones (‘are birds free from the chains of the skyway’) as a springboard for a largely new song. How far that’s plagiarism is a matter of opinion, especially when the original is presumably not covered by copyright anyway. He went on to be completely original though, after that initial phase of copying others.