Deuteronomy 17-19

In 17:7 God made it clear that the first stones of an execution were to be cast by the witnesses of tha capital crime.  So what is Jesus doing in John 8:7 when he charges the sinless to throw the first stones? Was he changing the Law? Was he accusing the woman's accusers of false testamony? Maybe he was pointing out to the Pharisees that their focus should have been on reforming the woman's life instead of rushing to condenm her.
Stuart