How has afterburner design changed since its inception?
This article suggests that they have. To name a few:

alluding to improvements in fuel diffusion
adding additional igniters downstream of the first to even out the burn
vectoring improvements to better keep the superheated gasses in the center and avoid overheating the structure at the sides
introducing cool air in a ring around the superheated exhaust stream to aid in a smaller thermal footprint
incorporation of stealth materials