Imam al-Dani in his “Jaami’ al-Bayan fi al-Qira’at as-Saba’”, vol. 2, p. 860, said:

And the Imams of the Qira’at did not choose the Ahruf from the Qur’an on the basis of what is more linguistically prevalent [in usage] and more analogically appropriate in Arabic, but rather on the basis of what is more established in tradition and most authentically transmitted. And if the tradition is established, then it cannot be rejected on the basis of analogical deductions in Arabic or what is deemed more prevalent in the language. This is because the Qira’a is a Sunnah that is meant to be followed and we are obligated to accept it and adhere to it.

الإمام الداني في جامع البيان في
القراءات السبع ٢/ ٨٦٠:

وأئمة القراءة لا تعمل في شيء من حروف القرآن على [الأفشى] في اللغة، والأقيس في العربية، بل على الأثبت في الأثر، والأصحّ في النقل، والرواية إذا ثبتت لا يردّها قياس عربية ولا فشوّ لغة؛ لأن القراءة سنّة متّبعة يلزم قبولها والمصير إليها.

Abu Ja’far an-Nahhas (d. 338 A.H.), said in his book “‘Irab al-Qur’an”:

السلامة عند أهل الدين أنه إذا صحت القراءتان عن الجماعة ألا يقال: أحدهما أجود; لأنهما جميعا عن النبي صلى الله عليه وسلم، فيأثم من قال ذلك; وكان رؤساء الصحابة رضي الله عنهم ينكرون مثل هذا.

The path of safety for the people of (this) religion, when there are two Qira’ahs authentically (narrated) to the Jamā’ah, is to refrain from saying that one of them is better (or more superior) than the other, for both of them came from the Prophet (peace be upon him). The one who says this is sinful, and the heads of the Companions (may Allah be pleased with them) used to censure this.

Abu ‘Ubayd al-Qasim (d. 224 A.H) in his book “Fada’il al-Qur’an” said:

“We only saw that the reciters recited the Qira’at to those who have knowledge concerning it, and then they stuck firmly to what they knew of it out of fear that they would deviate from that which is between the two slates [i.e. the Qur’an] by either addition or subtraction.

Thus, they abandoned all Qira’at which opposed what was written (in the Mushaf). They paid no heed to the Arabic linguistic schools concerning them if they opposed the script of the Mushaf, even if the [recitation based on] an Arabic ruling was more clear than [what] the script [accommodated].

They [i.e. the reciters] considered the letters of the Mushaf and preserving them like the well-established Sunan (Prophetic ways) which no one is allowed to overstep.” (p: 361).

” وإنما نرى القراء عرضوا القراءة على أهل المعرفة بها ، ثم تمسكوا بما علموا منها؛ مخافة أن يَزيغوا عما بين اللوحين بزيادة أو نقصان .

ولذا تركوا سائر القراءات التي تخالف الكتاب، ولم يلتفتوا إلى مذاهب العربية فيها ، إذا خالف ذلك خط المصحف، وإن كانت العربية فيه أظهر بيانا من الخط .

ورأوا تتبع حروف المصاحف، وحفظها ؛ كالسنن القائمة التي لا يجوز لأحد أن يتعداها”. انتهى من “فضائل القرآن”. (ص: 361).

Allah Almighty says: “And when Our verses are recited to them as clear evidences, those who do not expect the meeting with Us say, “Bring us a Qur’an other than this or change it.” Say, [O Muhammad], “It is not for me to change it on my own accord. I only follow what is revealed to me. Indeed I fear, if I should disobey my Lord, the punishment of a tremendous Day.”
(Surah Yunus: 15)

azZarqaani (may Allah have mercy upon him) said:

“There is nothing to be said after God and His messenger have spoken. Thus, this nation has consensus that there is no involvement of any man in the Nazm (words and word order) of this Quran, not in its style, its phrases, nor even the rules of its recitation. So whoever deviates from this consensus and follows a path other than the path of the believers then God leaves him to what he has chosen, makes his abode the hellfire, and what a horrible destination that is.

And here, we have seen the Quran in this verse prohibiting the Prophet from attempting such a thing in the most stringent manner, tying it to a severe threat; accompanying a painful retribution. So it’s impossible for Ibn Masood or anyone greater than Ibn Masood after this to change a single word from the Quran with a word of his own liking”.

((وَإِذَا تُتۡلَىٰ عَلَیۡهِمۡ ءَایَاتُنَا بَیِّنَـٰتࣲ قَالَ ٱلَّذِینَ لَا یَرۡجُونَ لِقَاۤءَنَا ٱئۡتِ بِقُرۡءَانٍ غَیۡرِ هَـٰذَاۤ أَوۡ بَدِّلۡهُۚ قُلۡ مَا یَكُونُ لِیۤ أَنۡ أُبَدِّلَهُۥ مِن تِلۡقَاۤىِٕ نَفۡسِیۤۖ إِنۡ أَتَّبِعُ إِلَّا مَا یُوحَىٰۤ إِلَیَّ إِنِّیۤ أَخَافُ إِنۡ عَصَیۡتُ رَبِّی عَذَابَ یَوۡمٍ عَظِیمࣲ)).

قال الزرقاني رحمه الله :
وليس بعد كلام الله ورسوله كلام ، كذلك أجمعت الأمة على أنه لا مدخل لبشر في نظم هذا القرآن ، لا من ناحية أسلوبه ، ولا من ناحية ألفاظه ، بل ولا من ناحية قانون أدائه ، فمَن يخرج على هذا الإجماع ، ويتبع غير سبيل المؤمنين : يولِّه الله ما تولى ، ويصله جهنم ، وساءت مصيراً .
وها نحن أولاء قد رأينا القرآن في تلك الآية يمنع الرسول من محاولة ذلك منعاً باتّاً ، مشفوعاً بالوعيد الشديد ، ومصحوباً بالعقاب الأليم ، فما يكون لابن مسعود ، ولا لأكبر من ابن مسعود بعد هذا أن يبدِّل لفظاً من ألفاظ القرآن بلفظٍ من تلقاء نفسه .
“مناهل العرفان في علوم القرآن” (1/188) .

Ibn Al-Mukandar said to Abu Hazim: “O Abu Hazim, there are so many people who meet me and ask Allah to give me blessings!
I do not even know them, and I have never done anything for them (yet they do this)!

Abu Hazim replied by saying: “Do not think that (their dua for you and their show of love) is because of (your actions). Rather it is (all) from Allah, so thank him!

Then he recited:

((Indeed, those who have believed and done righteous deeds – the Most Merciful will appoint for them affection.))
alQur’an: 19:96

Abu Ishaq said: “I saw a man asking alAswad bin Yazid, when he was teaching Qur’an in the masjid: “How do recite the verse – (fa hal min muddakir), with a (daal) or a (dhaal)?” He said: “With a (daal)”. I heard Abdullah bin Masood say: “I heard the Messenger of Allah (pbuh) say: “(muddakir) with a (daal)”.” (Muslim).

This narration (and there are countless others similar to them) demonstrates to us that the wordings and verses of the Qur’an have been preserved, and are ONLY accessible to us through the qiraa’aat that have reached us today via the recitations of large groups of people, across generations, without any letters being altered.