With Texas A&M on its way out, the Big 12 began negotiations with a replacement team from Texas in October 2011, TCU. This was despite the fact that the Horned Frogs planned to leave the Mountain West for the Big East the following year.
The Big 12 voted unanimously to accept TCU soon after, with TCU reciprocating.
Also in October 2011, the Big 12 needed to find a replacement for Missouri, and found one in West Virginia. It took a WVU lawsuit against the Big East for the Mountaineers to switch the next season.
By summer 2012, West Virginia and TCU were official Big 12 members. Also during the football offseason that year, the Big 12 hired current conference commissioner, Bob Bowlsby.
The Big 12's additions also had ramifications on other conferences: The Big East initially looked in all directions (including westward to San Diego and Boise) for new football members before becoming a basketball focused conference, while Conference USA and the Mountain West dissolved their conferences to form Tulsa's current conference, the American Athletic.