Umar bin Abd al-Aziz (may Allah have mercy on him) said: Breaking open a clay seal is very easy for me, and changing a judgement I have made is easier still, when I see that the truth is contrary to it.
Chapter: A ruler who exercises ijtihad, and then his ijtihad, or the ijtihad of someone else, changes in a matter where ijtihad is permissible: what he judged is not overturned
"When a ruler, or someone else, exercises ijtihad in permissible cases and then his ijtihad changes, it is not permissible to change the judgement."
By way of evidence from what has passed concerning an error in the qibla in the Book of Prayer.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20373