Wait a minute. Jon thought over what he said. He had wish that this would last forever. Then he made a mistake in fixing the wish--he said that he wished for "this" to mean... what did he say... he said "this body's so great".
So now, this body would be great forever. The wish didn't say anything about him being stuck in the body. And, he thought, it certainly was a great body, so if the body stays that way forever--whether or not he's still in it--the wish was true.
Still, that didn't solve his first problem. He was stuck in Sarah's body until after the test.
