Hisham (may Allah have mercy on him) narrates from al-Hasan (may Allah have mercy on him) that when a man makes a bequest, he may change his bequest whenever he wishes. It was said: "(Even) emancipation?" He said: "Emancipation or other than emancipation."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المدبر / 21569