Ninner,
Yes, the slotted BAS on the rear of the bolt can be replaced with the more recent hex version. I just completed the exact swap you are asking about on a flat back long action chambered in 7 Mag. It also had a three screw factory trigger identical to the one in your photo. I didn't have a parallel bladed screw driver large enough to fit the slot, so I put multiple layers of painters tape on the largest flat blade screwdriver to create tight fit. It turned off very easily compared to the recent battle I had with hex BAS that must have been factory assembled by a gorilla. If that didn't work, I was prepared to use the edge of a 12-inch piece of spare bar stock I had lying around that fit the slot perfectly. Good luck!