Of course they are going to change and refine things before the first race, based on the data gathered during the tests. It would be disappointing and quite worrying otherwise.
But some components are homologated and others cannot be changed that quickly. Also, I don't think that having a "common crash structure" and compromising other components around the rear suspension (in order to have the opportunity to fit both a push-rod and a pull-rod) will help them make the best car possible because, as a result, not all of the components will be optimized, and the competition is stronger than ever. Surely a middle ground solution will not make the best packaging.