I always thought that witches can control the changes in their aspect while heirs had no control of how their aspect changes themselves

It really depends, specially, on how successful of a Witch or Heir they are. John can poof into wind at will while Jade couldn’t control being made part Dog, for example! On the contrary, John’s Windy Thing activated by themselves without his control several times, while Jade actively used them to Change the Size of things. The two Classes do need a balance about how they change things and how it all changes them.