is there a specific reason crowley's "look" changed from a flashy 20yo lawyer to middle-aged washed-up rock star or was that just for the sake of having an actor in their late 40s and needing to adapt to that?

I’m not quite sure where you get a twenty year old lawyer from. The book says,

Crowley had dark hair, and good cheekbones, and he was wearing snakeskin shoes, or at least presumably he was wearing shoes, and he could do really weird things with his tongue.

Crowley’s hair is still dark, although it’s a dark red, and David definitely has good cheekbones.

(R.P. Tyler calls him “young man” in both the book and the show, but that doesn’t mean he looks twenty. It just means R.P. Tyler is that sort of person.)

We wanted a look for Crowley eleven years ago that distinguished him at a glance from Crowley now. David rejected the idea of a suit, and worked up the Crowley clothes, then and now, with costume designer Claire Anderson. You can see the original clothes design in the Good Omens TV Companion book.