It wouldn't be the Big XII. It would be a new conference that UT is trying to put together. Go read my link above, check out the source and see what he is saying about what is going on. He talks about how he needs to hear the same story from 3 reliable sources before he will go public with it. I certainly don't know that he is any more credible than anyone else but it seems he at leas has some credible connections based on his time in the business.
So Texas is going to poach multiple teams from multiple power 5 leagues to form a whole new 20-team conference? Yeah, there's a better chance of Nebraska coming back on their own to an Oklahoma-less Big 12.