I don't have Heart of the Swarm, and I don't plan on purchasing it in the immediate future. However, I know that some of my Battle.net friends have gotten it. Will they still be able to play Wings of Liberty matches with me, dropping all of the changes made by the expansion?
Edit: If nothing else, I would love to be able to play custom games with my friends, even if we can't play in the leagues, since that's all I did with them when we played.