How to change your expansion pack after the recent update?

The option in the "gameplay" settings is no longer available..

Solution 1:

Go to custom. Click melee. You will see two droplist. The left droplist let you pick which expansion you want to play with.

This change was made because of going free to play for most contents. So it no longer make sense to still have a gameplay option.

Source: http://us.battle.net/sc2/en/blog/21183638

The Wings of Liberty, Heart of the Swarm, and Legacy of the Void ladders have now been consolidated into a single ladder using units, balance, and data from Legacy of the Void.

The Expansion Level selection for Wings of Liberty or Heart of the Swarm balance games has been moved to Custom > Melee.