i did something similar to what you suggested.
i split the clip where the effect started. there i defocused it slightly and also scaled it a bit smaller to give the impression that the lens was disengaged (i did this with a straight cut rather than a dissolve). over the defocused clip i placed a circular matte that a few frames later rotated out of frame to simulate the lens being moved out of place (which also created the vignette look), with another circular matte moving into place a few frames later revealing the new shot (which was also scaled back slighty & defouced). a couple frames later that image popped back into focus and 100% scale.
what i didn’t do was move the actual image, as from my recollection the image didn’t move when you changed a lens on a bolex.
i have a small quicktime if you’re interested in seeing it https://s239.photobucket.com/albums/ff86/athunt/?action=view¤t=lenschangetest2.flv