Even before the Disney merger there were diferent LEVELS of canon. If something happens in one canon that contradicts another, the higher canon supersedes the lower level canon. It has always been that something in the expanded universe (say a comic) can be considered canon unless/until something in the movies contradicted it, in which case the films (being a higher level of canon) supersedes the EU content. They rank as follows.

G Canon - This would include the films I-III, IV-VI, and VII-IX.

T Canon - Television shows such as The Clone Wars and Rebels

C Canon - EU elements such as comic books and video games that have not been contradicted by a higher canon.

S Canon - Also EU, but has been contradicted by a higher canon. Some elements of these stories which have not been contradicted may still be considered a part of the SW universe.

N Canon - Any material that is directly contradicted by higher canons. This would include things like "what if" stories.

I believe this ranking of canon was scrapped with the Lucasfilm/Disney deal, however I see no reason why it should not still be a viable way to determine canon and I will continue to use it. It's nice in that you can still use the EU where it isn't contradicting the movies, and where it does, you can simply say that story exists in a parallel universe. =P