You could call it Elegant Variation: https://en.wikipedia.org/wiki/Elegant_variation

In the case where it's rather forced, you could call it Inelegant Variation instead.

Exactly that, thank you!

I realize no one is claiming otherwise, but this writer almost certainly did it intentionally for humorous effect. ("Aubergine wyrm" is just too good to be accidental.) It's like "the lick" in jazz.

Using modern terminology, I think it's actually a drake, not a wyrm. Wyrm and dragon were only a synonyms centuries ago, nowadays dragon is an umbrella term for those and more.

Some people want to be artists but are journalists instead.