TMZ broke the story ... Quincy was subpoenaed to testify in Steve Wynn's defamation lawsuit against Francis, in which the "GGW" creator has claimed Wynn threatened to kill him and have him buried in the desert over a debt Francis owed to Wynn's casino.

According to Francis, Quincy had seen an email from Wynn detailing the alleged murder plot -- so Wynn called on 79-year-old Quincy to testify in order to show Francis was lying.

Quincy claimed he was too sick to testify -- claiming he experienced heart palpitations and shortness of breath after getting served with a subpoena -- and the judge agreed.

But now Quincy has changed his mind -- we're told he WILL testify against Francis because he believes it's the right thing to do.

Sources tell us Quincy will tell the jury he NEVER saw any email from Steve Wynn about an alleged murder plot -- and he wants to set the record straight in the name of justice.