The question is: will the 831 caps fit on the 853 bodies? It might solve the 853 omni shortage problem.
I haven't tried it, but I would guess they do. They have the same thread. But, remember that AT831 are cards, not omni. I think AT803 is the omni version similar to the AT831, but I have never actually seen one of these or tried switching them around.
My informal experience (so far) is that the capsules are what make the difference, so putting (cheap, widely available) AT853 caps might be a nice option for people who want to improve an AT831 setup. I was skeptical, but having tried both, I now see that 853s are a lot nicer sounding than the 831s. I bet there is a different diaphram in there.
As for getting omnis, I would like to know the definitive answer. Are they discontinued, or just rare? Perhaps someone should get the true answer from AT.
Richard