Have an in-between frame always, or it'll be hard to communicate. The more the better, but it's best to keep them all in the same page so the reader doesn't lose track of things - too much can feel slow but go for what keeps the overall pacing. It also depends on the method the character uses - if it's a sci-fi device, maybe have them split apart in pieces of hologram-like structures; if it's magic, have their lineart altered and glowing to show they're shifting out of reality, or a character that's moving really fast can be lines that replace where their body would be. Shapeshifting is similar - a character that morphs into a different form might transition and slowly change each body part, or appear with the new shape in a puff of smoke.
I actually have examples for both from my comic!