Would you overlay new ones, or scrape/rub/??? the old ones off first?

As far as overlaying: exact fit is one thing, but if the old ones are cracked or peeling or really oxidized, that texture would tend to show through the new ones.
Removing the old ones without paint damage could be an issue if they're really sun-baked on there.
Least invasive removal method is a heat gun and a lot of patience. Leftover adhesive you'll have to experiment with: rubber wheel, adhesive remover or lacquer thinner, depends on type & condition of paint.

Oh jeez don’t over lay. Listen to topside, good advice. With a little heat most decals come off.