I had the exact same problem with my RRA lower. The detent was so tight in the hole that it would not allow the selector to move. And when I disassembled it I couldn't get the detent out and still couldn't move the selector. I tried everything!!
Here's what worked for me. I applied oil into the detent hole and let sit for a few minutes, to hopefully allow it to penetrate and lubricate the detent and channel. I then used a punch (covered with tape so as not to mar the finish) and hammer and forcefully drove the selector out. I had to give it a couple of decent whacks, but it came right out. The force of the selector being driven out pushed the detent out the bottom of the hole. Neither part was damaged at all, nor was the receiver. Be sure and catch the detent as it will come flying out! I then used some fine grit paper on the detent and a handturned drill bit in the channel to remove any burrs and smooth the passage. Applied a drop of oil to the channel, reassembled and everything works fine!
This may seem severe, but I mean I tried everything I could think of! Good luck!
Hershey