@ninja8tyu shouldn't be that hard, just stick to the same tonic notes, and melodic phrase and progression and add different rhythm, and passing notes. Not gonna sound great but it'll be different. You can even switch up the key. Mary had a little lamb. House/dub/heavy metal remix.
@ninja8tyu yeah, being limited by vocals forces a certain rhythm+tempo unless you can do some melodyne magic I haven't learnt yet. Think of those Anime MAD's. They pitch shift the vocals to match the song. https://m.youtube.com/watch?v=iSMB0YNJC0g