Or possibly try for the look they used in the League of Extraordinary Gentlemen when Jeckyll and Hyde changed by having certain parts of the character bulge out (using endomorphs - just scale parts of the model).
Doing this, you can simply distort parts of the model and have the camera snap to extreme close ups for a few frames whilst quietly switching models in the background. (Perhaps have the other model in on a 100% dissolve tracking the motions of its sibling - that way, during the change you can also flash-swap them for small instants)
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