After the death of Muhammad, the Quran was changed?
This is false. The first three Caliphs, especially Uthman completed a critical analysis of the entire Quran text.
This was due to the rapid expansion of Islam in the first decade amongst people whose native language was not Arabic. So it was deemed essential the text retained its original purity.
Uthman sent copies of the standardized Quran text to the centre of the Islamic Empire. Today, copies attributed to Uthman exist can be found in Tashkent and Istanbul.
Apart from a few dialect variations in copying, the oldest documents known to the present day found throughout the Islamic world are identical; the same is true for documents preserved in Europe (fragments in the Bibliothèque Nationale in Paris which, according to the experts date from the 8th Century AD).
The minor dialect variations do not change the general meaning of the text. For example, the absence of diacritical marks could make a verb either active or passive and in some instances, masculine or feminine. This is a minor issue since the context indicated the meaning in many instances.
Therefore the Quran has been fully preserved since its revelation.