Is the supplication, "O Allah, if You decree evil deeds for us, then transform them into good deeds," attributed to 'Umar ibn al-Khattab, authentic?

This narration attributed to Umar ibn al-Khattab, may Allah be pleased with him, was not transmitted with the wording mentioned in the question. The correct wording of his supplication was: "O Allah, if You have written me among the people of felicity, then affirm me therein. And if You have decreed upon me sin and wretchedness, then erase it from me and affirm me among the people of felicity. For indeed, You erase what You wish and You affirm, and with You is the Mother of the Book." Its chain of transmission is hasan (good).

Related to this supplication is the question of whether what has been decreed for a servant's destiny can change. The verified answer is that Allah writes a lifespan for the servant in the scrolls of the angels, and it may increase or decrease based on deeds. Shaykh al-Islam stated that erasing and affirming occurs in the scrolls of the angels, not in Allah's knowledge or the Preserved Tablet (al-Lawh al-Mahfuz), which does not change. This supplication indicates that Allah is able to change what has been decreed for the servant, whether wretchedness or felicity, by His will.

As for replacing evil deeds with good deeds after they have occurred, this happens through repentance and righteous actions, as stated in Allah's Almighty words: ﴿Except for those who repent, believe and do righteous deeds. For them Allah will replace their evil deeds with good ones. And ever is Allah Forgiving and Merciful.﴾