I agree, neither are nice.
Not my place to dictate.
*change_units* is ambiguous as to whether it affects the payload.
I suggest one of these:
 - *convert()* and *converted()*
 - *convert_units()* and *converted_units()*

I was going to say next, *"But anything that's consistent with the other funcs (collapse and collpased)"*
but I see we don't have *cube.collapse*. I wonder if that same logic applies here?...