Rashi (in Bo): Hashem turned the bloodshed into the blood of B'ris Milah ¹ - when Yehoshus circumcised the entire nation upon entering Eretz Yisrael. ²

Bechor Shor (Bereishis 6:7): There are three kinds of retraction. Sometimes a person promises, and could fulfill, but does not want to - this is Ish vi'Chazev. Sometimes he thinks that he can fulfill, but he errs, and b'Ones he cannot - u'Ven Adam v'Yisnecham. Hashem does retract when he decrees evil, e.g. on Ninveh, and they repent, or if He decreed good, and later man sins. ¹ The retraction is due to man's change.