"I wish that tomorrow you woke up as my new younger sister Roxanne. I wish Roxanne to be essentially just like me at my original age but red-headed, smaller by half a foot and really self-conscious of the size of the soft ass she inherited from dad's side of the family."
The essence of a thing is the basic, real, and invariable nature of a thing or its significant individual feature or features. Like would refer to having the same or similar characteristics/qualities.
There were many ways such a wish could be interpreted.
The most Jon could hope for was that, as the wish referenced physical features as differences between Roxanne and Sarah...that the similarities were of a physical nature.
The worst case scenario was Jon would wake up in what was essentially...stuck in a pseudo-Sarah body with Sarah's life.
There was an out though...as Sarah hadn't specified one thing in her wish.
Of course, wish dynamics were beyond Jon's ability to influence, and he was too overwhelmed with worrying about being female(something that was a certainty) with worrying about how much worse it could be.
