Nothing happened. The opposite of not needing a wheelchair was needing a wheelchair.
"Is that guy the owner?" whispered Jon, as he and Karyn tried to sneak behind the men.
"Has to be," replied Karyn. "Remember, the owner wished nobody could take it from him. That guy in the wheelchair took it away from the other man. So the other man can't be the owner. The guy in the wheelchair must be the owner."
"Damn it!" said the man. "I wish I knew why that didn't work!" Of course, reversed, that meant he had no idea why it didn't work at all, so again there seemed to be no change.
"I wish I was a healthy man!"
Which caused him to shrink, as the reversed stone to turn him into a handicapped 11 year old girl. He was already in a wheelchair, but as he turned female he realized something was drastically wrong with the wish.
"This wish didn't work!" she yelled in her suddenly much higher voice. "This damn stone made me a girl! And I still can't even get up from this chair!"
"What do we do, Boss?" asked the other man.
"We go to the other location, just as Ms. Summers said."
The girl in the wheelchair rolled away, accompanied by the three men (the two from the warehouse, and the one that was with her). Meanwhile, Jon whispered, "I wish that Karyn and I were invisible until we change back," and followed them.
The new location was only a block away, but unlike the warehouse it was a public place--right outside a McDonalds. It would be much harder to try any funny stuff there, like bringing in snipers or extra thugs or just trying to kidnap Ms. Summers.
Jon and Karyn watched, invisibly, Jon clutching his own stone, as the three men and the newly transformed young girl in the wheelchair met the people they were negotiating with. They seemed to be around college age. The leader was, oddly, a frail-looking blonde. Accompanying her was a man her age, a redheaded woman, and an older man in a leather jacket with bleached, spiked, hair....
"Hello," said the blonde. "My name is Buffy Summers. I understand you have a magical artifact we might be interested in?"