What changed was the release of drilling data the reveled shocked quartz and other types of damage to multiple types of rock that doesn't happen naturally with earth's forced existed over a very wide area and to incredible depth. This data was the smoking gun that showed a huge impact crater did indeed exist, when previously we had suspicion but no proof of an impact.

And it gained even more traction when they did some more drilling to pinpoint the central peak, early 2010's I think.

Prior to the drilling data release, all they knew was there was ejecta all over the peninsula.