My understanding was Joe Looney was working as the first team LG at OTA's. Then he hurt his back, forcing the Cowboys to use Jonathan Cooper, Emmett Cleary, and then Chaz Green at the position.
Cooper was working for a while at center, probably to see if he could replace Looney as the three-spot gameday backup if, indeed, Looney ends up winning the job. I think the staff is privately pulling for Cooper to step up and win the spot, but he just hasn't been the player everybody thought he would be.
As for Green, I can't see him as a guard. He just doesn't play with power. He's probably the swing tackle once again.