...Or you could just unscrew the lens from the adapter, leaving the adapter on the body? This is what I always do when swapping one M42 lens for another on my 50D.
You also get pretty reliable metering with any lens on a Canon body, in either M or Av mode. IIRC you can't use the Av mode on most (all?) Nikons unless the lens has the electrical contacts in it to talk to the body. I may be wrong You are, as I stated earlier in the Thread the Nikon D200 and up can use AI and AIS fit lenses in both Av and M open aperture modes as they have an AI (auto indexing) ring (see image below) as I pretty much only use Canon but I've found my 50D very, very versatile in terms of what I can mount on it. I have lenses ranging from a 28mm f/2.8 Nikon to a Kaligar 135mm f2.8 (a little soft wide open but crazy sharp when stopped down to f/5.6, almost puts my Canon 50mm f/1.8 to shame) to a Optimax 300mm f/5.5 (impossible to MF, although easier than my Tokina 500mm f/8 mirror lens).