When making morphs, I found it useful to make the most dramatic changes to the geometry in Modo and then smooth things out in Zbrush. With Zbrush alone, you could use the hook tool with symmetry and then pull things back into shape and smooth areas to get rid of the distortions. If you dropped to a lower level of subdivision as the first step, you might be able to make transformations to the lips region with greater ease.

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