Lyanna is exposed as the Knight of the Laughing Tree after winning that round at Harrenhal. does this change anything?

Depends on what Aerys II does. If he tries to imprison Lyanna over this, you might well have a stampede into the Southron Ambitions camp or Rhaegar’s Great Council camp, because there’s no argument from the Aerys loyalist camp that will fly, especially with chivalric passions at full flood. Imagine Rhaegar’s reform agenda linked to a public image as the defender of innocent young women and the knight’s oath. 

If he doesn’t, then it’s a minor scandal – Lyanna Stark gains a reputation as being a headstrong wild Northern woman which makes her a celebrity on par with Ashara Dayne and Cersei Lannister, but I doubt Robert drops the engagement over it. My guess would be that he simply changes his mental ideal, from Lyanna as Woman On Pedestal to Lyanna as Nymeria Reborn.