In Neumann's KM 180-series microphones, the amplifiers are all absolutely identical in terms of their circuitry and capsules are removeable (mainly for repair purposes), but the near-hemispherical plastic part behind the capsule is shaped differently for the three different models. So although you might be able to obtain capsule heads for the KM 183 and KM 185 as replacement parts if you made enough phone calls, the KM 185 capsule head wouldn't be in its proper acoustical setting if you put it onto your KM 184 body, and wouldn't have the specified high-frequency response.
The KM 183 capsule, on the other hand, has a sealed backplate and doesn't care what is or isn't behind it. So it's like having the blood type that's the universal donor (O-negative?)--that capsule can go on any of the bodies and work correctly. But the reverse isn't true, because the series simply wasn't designed for interchanging the capsules. For that you need the KM 100 series, which has seven interchangeable capsule types if I recall correctly.
--best regards