The Big 12 has sent a letter to Texas A&M which outlines the procedures required in order for A&M to depart the conference. Neither side would release the letter (as of yet) but you can expect that the letter had been drafted weeks ago and was ready to go on short notice.
The expectation is that A&M will face financial penalties for leaving, a result that A&M can accept unless those penalties stem from the 13 year agreement with Fox which has a potential price tag of $1 billion.
How that contract ultimately gets resolved or restructured could go a long way to determining the likelihood and the speed with which A&M will be invited to join the SEC.

