What Changes have Happened to the Pali Canon? In the 6th council changes have been made to the Disclosure in Establishing Mindfulness in the Middle Length Discourses according to Ven Sujato and Ven Analayo where the section on the 4 Noble Truths was expanded. 3rd Council the Abhidhamma as made officially part of the canon.

Is there other known instances where changes to the Pali Canon happened either at this council or elsewhere?
The Scriptures or the Doctrine of the Supreme Buddha is called the Sutta Pitaka, and consists chiefly of instructive discourses delivered by the Buddha to both the Sangha and the laity on various occasions. A few discourses, expounded by disciples such as the Venerable Sariputta, Moggallana, and Ananda, are incorporated and are accorded as much veneration as the Word of the Buddha Himself, since they were approved by Him. 

The Sutta Pitaka consists of the following five Nikayas(Collections):


Digha Nikaya (Collection of Long Discourses)
Majjhima Nikaya (Collection of Middle-length
Discourses)
Samyutta Nikaya (Collection of Kindred Sayings)
Anguttara Nikaya (Collection of Gradual Sayings)
Khuddaka Nikaya (Smaller Collection)


There are no changes made to the first four, but the fifth got expanded with time. Khuddaka Nikaya, the fifth, is a minor or smaller collection. Although termed "smaller", it is in fact the largest as more and more books have been added to it over the years. It has grown to 15 books in the Thai and Sri Lankan versions. In 1956, the Sangha Council in Burma added another 3 books, which are not the Supreme Buddha's own words. These 3 additions are additions are Questions of King Milinda, Petakopadesa and Nettipakarana. This is how the Khuddaka Nikaya grew from a minor collection to become a major collection! In the future, say in 500 or 1000 years' time, this would definitely create more confusion. Out of the 18 books now, probably only 6 are reliable in that they do not contradict the earliest 4 Nikayas. These 6 reliable books are the Dhammapada, Sutta Nipata, Theragatha, Therigatha, Itivuttaka and Udana.. This is how the Khuddaka Nikaya grew from a minor collection to become a major collection! In the future, say in 500 or 1000 years' time, this would definitely create more confusion. Out of the 18 books now, probably only 6 are reliable in that they do not contradict the earliest 4 Nikayas. These 6 reliable books are the Dhammapada, Sutta Nipata, Theragatha, Therigatha, Itivuttaka and Udana.

But I personally would trust the Questions of King Milinda and Nettipakarana a whole lot more than the Abhidhamma, which got added much later on. I will not go into Abhidhamma (the third basket of the Tipitaka) as it will confuse things in my humble openion. When there are nearly 18,000 suttas that are found in the canonical texts why would one want to check the Abhidhamma? It came only after the 3rd Buddhist Council. So my advice to you is to please limit yourself to reading only canonical texts (Sutta Pitaka) until you are fully established in the Path. Do not go even into the Visuddhimagga – it is only a thesis that Buddhagosha Thera wrote to show his qualifications.