That's where an animator can get very creative choosing what they want that "Morph" to be. Studying "smear" animation is a great way to get an idea of even how to morph 2 very different objects into one another. Sometimes objects will stretch, blur or even bounce into place to create the desired effect, it's honestly a lot of fun to put together!
I myself teach traditional frame by frame animation, as well as puppet animation using various animation software including Krita and Clip Studio Paint, and I find some of these creative methods to be some of the more exciting parts of the process.