I replaced the front two a few months ago. No new codes since.
I knew that the rear sensors just checked the cat thats why I was in no hurry to fix it. But since the front control the fuel mix I will want to do that sooner. I will proabley do as you said, now I just have to find time...sigh .Buy two new sensors, and put them in the front of each back. Then take the one good front one and put it in place of the bad rear one. The front sensors control fueling, and the rear sensors only monitor the health off the cat.