As speculated earlier, Williams was expected, but Anderson and Brisiel open up roster spots for Jarvis Green and DeMario Pressley. No word yet on the 53rd man.
The person who could most benefit from this move is WR Dorin Dickerson as he moves up the depth chart and could see time as a receiver in place of Anderson.
EDIT: In addition to DL Jarvis Green being signed earlier today, the Texans have signed WR Derrick Townsel from their practice squad. Because of the two vacancies left by Townsel and the defection of DT Malcolm Sheppard, the Texans also signed C Brett Helms and WR Montez Billings to the practice squad.
SB Nation Houston and Battle Red Blog will have more on the story as it develops.