What is the ruling on one who changes a single word in a supplication taken from the Holy Qur’an, whether intentionally or unintentionally?

There is no objection to changing some of the wordings of the Quranic supplications, as long as it is not intended to distort the Quran, but rather to supplicate in a manner appropriate to the situation of those being supplicated for, such as someone who changes a singular pronoun to a dual or plural pronoun. Evidence for this is the Prophet's (peace be upon him) practice in commencing prayer, where he would supplicate with wordings in which some Quranic supplications would change, as in his saying: "O Allah, You are the King, there is no god but You. You are my Lord and I am Your servant. I have wronged myself and acknowledged my sin, so forgive all my sins, for indeed none forgives sins except You. Guide me to the best of morals, for none guides to the best of them except You, and turn away from me the evil of them, for none turns away from the evil of them except You. I respond to Your call, and all good is in Your hands, and evil is not attributed to You. I am dependent on You and return to You. Blessed and Exalted are You. I seek Your forgiveness and repent to You." Also, evidence for this is his teaching one of his companions to say: "O Allah, grant us good in this world and good in the Hereafter, and protect us from the punishment of the Fire," instead of his supplication in which he used to say: "O Allah, whatever punishment You intend for me in the Hereafter, hasten it for me in this world."