What is the first DECISIVE use of a cannon to overcome a castles
defences. I am seeking an example of new technology overwhelming old
technology and thereby changing the rules of the game. Examples other
than the canon are welcome, but the must be monumental. By FIRST I
mean well documented.

Greetings Kongulu, 
I'm not sure if this answers your question exactly, but the first
documented use of success with a cannon seems to be the Battle of
Odruik. Here, the Duke of Burgundy's cannons broke down a wall, which
was the first ever wall to succumb to cannon fire.

Trebuchet: The Cannon
http://web.grinnell.edu/techstudies/vick/gun.html

Medieval Life and the Hundred Years War - Sieges
http://www.hyw.com/Books/History/Sieges.htm

Medieval Technology Pages - Cannon
http://scholar.chem.nyu.edu/tekpages/cannon.html


Another Example worthy of note:

Defending and Capturing a Castle - Lady Sierra's medieval times
http://www.geocities.com/Athens/Acropolis/2025/defen.htm
- Probably the first well-known major siege victory using cannon was
in 1415, when Henry V attacked Harfleur in Normandy, and captured it
in less than a month, and that set the trend to prefer cannons in
sieges.

Search terms used
first cannon siege
first successful cannon siege
first castle gun siege

I hope this has been a most helpful answer. If you have any problem
with it, do please post a Request for Clarification and I shall
respond. Thank you.

Thank you.

An alternative is the invention of a rapid firing gun by Gatling and
Maxim.
"The Maxim Machine-Gun was adopted by the British Army in 1889. The
following year the Austrian, German, Italian, Swiss and Russian armies
also purchased Maxim's gun. The gun was first used by Britain`s
colonial forces in the Matabele war in 1893-94. In one engagement,
fifty soldiers fought off 5,000 Matabele warriors with just four Maxim
guns."
http://www.spartacus.schoolnet.co.uk/FWWmachinegun.htm
answerfinder-ga

Catapults and trebuchents (spelling?) were pretty effective at
knocking down walls even before cannons, so I'm not sure that cannons
were decisive in themselves.

Many thanks for the full rating and the tip! :)