Scarborough is reporting that Texas went almost exclusively with a 4-2-5 defense in the first half of spring practice. Gideon, Thomas, and Christian Scott at safety, A. Williams and Chykie Brown at corner and the usual suspects upfront.
The basis for the switch really comes from making Aaron Williams a full time cover corner opposite Chykie Brown and rotating Keenan Robinson down to the Sergio Kindle role. The switch should also tell you that the coaches feel better about Williams' size/speed combination than Curtis Brown's.
The base 4-2-5 is the best way to matchup against the spread offense assuming that you have great run support from the secondary and can get to the QB with four linemen. By having Christian Scott and Keenan Robinson on the field at the same time, you can change to something resembling a 4-3 by spinning Robinson down to an edge rusher and moving Scott into an SS/LB hybrid role. Additionally, Thomas's range and Scott's bonecrushing hits stand to have the best change of creating turnovers in the secondary.