Is multiplayer Heart of the Swarm backward compatible with Wings of Liberty?

Also be aware that Blizzard enabled "spawning" a while ago, which will automatically bring all players up to the highest level of anyone in their party. So if you play in a group with someone who has HoTS, you will have access to the HoTS ladder and units.

Heart of the Swarm players can play with Wings of Liberty Players, and even play on the Wings of Liberty match-made ladder, if they so choose.

However, if they want to play as if they did not have Heart of the Swarm, this must be manually toggled.

So don't despair! You can continue to play ladder matches and custom games with your friends -- you just have to "dumb down" to their level!