With nappy changes I find something abstract that DD hasn't seen before, like a tea strainer or a teapot lid or a bag of cotton balls and I have them ready to give to her to look at when her nappy is being changed. I tell her the rules about the new toy though:

"Look DD, look what mummy has for you! Its a teapot lid, wow, isn't that amazing. Ok you can have it, but you have to lie still ok? If you roll, mummy will take it off you. Ready, here it is."

And I do take it off her if she rolls, but I give her a warning first. "lie still bub, or Mummy will take it off you"

It seems to work. DD was doing the full body stiffening / arching thing too but not anymore.