How has Islam changed over time?

Islam is just another religion.

And like most religions, it has gone through many changes over the years and there have been many movements within the religion that have sought to reform it.

First major reformation - Quran

The earliest perhaps is the compilation of the Quran, that was done with the help of the first Caliphs - Abu Bakr and Umar. This involved transcribing the verses that Muhammad revealed orally. When Muhammad died, the Quran wasn't in the book form that we see today, it was a collection of various verses written on parchment, leaves, tablets and sometimes, even stone. In the reign of Abu Bakr and Umar, these were compiled together and put in a basic form.

However, this wasn't to the satisfaction of Caliph Usman (aka Uthman), who ordered the proper compilation of the Quran into the form that we see today. He ordered all older versions to be destroyed to ensure that all existing versions were alike [1]. This could be said to be the first real transformation of Islam.

It is claimed that the Quran we see today, is the exactly like the version that Usman commissioned. Although, it isn't impossible that there might be slight differences. A notable difference is that about a century after Hijra, diacritical marks were added to the text to aid non-Arab Muslims in the pronunciation of the words of the Quran. However, the text essentially is the same [2][3].

Second major reformation - Hadith

This is what the wikipedia page about Hadith says - 

Hadith (Arabic: حديث‎) in Muslim religious use is often translated as prophetic 'traditions', meaning the corpus of the reports of the teachings, deeds and sayings of the Islamic prophet Muhammad. The hadith literature was compiled from oral reports that were in circulation in society around the time of their compilation long after the death of Muhammad. Bukhari's collection is considered the most reliable by many traditional religious scholars who are Sunnis. The Shi'as believe in an entirely separate body of Hadith.

These directives are supposed to be followed by Muslims around the world and as such, any changes to the Hadith means a change in the way Islam is practiced. The interpretation and authenticity of hadith is one of the many questions on which the Sunni and Shi'a Islam differ. It is also one of the areas of Islam that leave a lot of room for interpretation, leading to differences. These differences are the basis of the formation of many different schools of thought in Islam.

Schools of thought in Islam

Similar to the Bhakti and Lingayat movements in Hinduism, we have Sufi, Quranist, Ibadist and Ahmadiyya movements in Islam [4].

Other than these social movements within Islam, there are also political movements like Salafism and Islamism which hold that Islam is not only a religion but a political systemthat should govern the legal, economic and social imperatives of the state [5].

In recent times, many movements for modernization of Islam have been started and there are also movements for Islamic feminism [6].

Reforms by rulers

Islam has been the state religion for a number of dynasties and depending on the ruler, Islam has also undergone major transformations. This in turn, has depended on how strictly, the tenets of Islam have been implemented and how literally, the verses of the Quran have been interpreted.

At times it has been tolerant and progressive, such as during the period of major scientific progress from the 8th to 14th century [7] or even during the period of extensive political reformation during Akbar's rule in India [8][9].

Then again, Islam has been made intolerant and hostile towards other religions (and indeed, towards different interpretations of Islam), such as during the reign of Aurangzeb, a descendant of Akbar himself [10]. Or when the Taliban were in Power in Afghanistan or even in the case of the Kingdom of Saudi Arabia.

Islam has changed many times since its inception. No question about it. -quora.com