Touhou arrangements having original lyrics and instrumentation but reusing the same old melodies is a lot like how hymns and ballads were adapted in past centuries. Like Greensleeves and Auld Lang Syne.

It feels like this was common up to the 21st century but modern
music considers it bad form and any new music like this must be labelled a 'parody'.

I was about to post and then I noticed >>564