Judging from jewelry the change doesn't have to be "substantial" to call it your own. If the copy doesn't fit in the mold exactly you're not going to win a lawsuit.
I'd look at hats like the Blues. You're doing the standard progressions that have been done a million times before.
Etiquette-wise, you know what's right and wrong.
If you set out to sound like BB King and claim it as your own you're doing wrong (and will be sued even if you don't set out to sound like him but that's a different topic ).
The people who took my dolphin ring and put the nose on the side of the tail instead of in the center did wrong, IF they got the idea from my design, I didn't invent dolphins, they're public domain, but I was the first one I ever saw that design from, my ring was not public domain.
The person who made a mold of my dolphin ring definitely did wrong.
So are you embellishing someone else's design or are you using someone else's design to get the basic public domain hat that you then embellish?