I've uploaded a scene file with an example transformation animation as a Public access post on my Boosty: https://boosty.to/deckardx08/posts/.....hare=post_link The effect is achieved mainly through 2 custom Geometry Nodes modifiers. "LocalTransforms" enables scaling an object relatively to an external dummy. Can be used after Armature modifier in the stack, and very useful for quick squash and stretch effects. "GlobalShapekeyBlend" functions similar to Shape Keys, with the main difference being that it is executed in world space, and can morph between two objects bound to different armatures. Additionally, dissolve shaders were used for some transitions. As always, done in Blender 3.2 and Eevee
General idea was to identify "visual hook" objects that persist between forms, and manipulate them using "GlobalShapekeyBlend". The rest of the transitions are to be achieved through careful keyframes and dissolve. Also, I've used Drivers extensively, so all the various modifier and shader parameters could be accessed from the same Armature