The article originally said that Su Yang (ie. Yang Su in Western order) was his "real name". The Nordic Go Dojo (see below) states that his "real name" is now Jeff Su. In 2021 he mainly goes by just Jeff.
Of course, it's likely still Su Yang in China but now Jeff Su in Finland (listed on the NGD site as his country of residence), and more broadly outside of China. It seems to me (bugcat) that JeffChang is really just a username (and so I've adjoined it to show that).
 According to the KGS Game Archives page, he created the JeffChang account in October 2005, reached 9d in January 2006, and abandoned it in August 2009. He (obviously) does have at least one other KGS account, but I (bugcat) don't know its name.
According to the GoSensations interview, he began studying Go at eight years old. In / over 2007 he studied in Europe.
Su Yang was one of the main teachers of Go'n'Games' 2005 China trip and has taught go for many years. He offers online lessons and offline game comments on KGS. You can contact him on his e-mail, email@example.com. 
 Added in 2007. There aren't that many people on Hotmail these days -- best to take this address with a pinch of salt. Jeff is on Discord now.
The May 2007 version of the article stated that he was 26 years old. In February 2012, this was "updated" to 28. So between August 2021 and January 2022, he would be either 40 or 37 depending on which version you believe.
Jeff is the most experienced of the NGD teachers, and is in fact the teacher of four of our teachers, Antti, Lukáš, Matias (Pankoke), and Oscar (Vázquez). Jeff learned the game in China as a child, and has now been teaching western go players for almost 20 years.
Jeff is famous for teaching the game through entertaining metaphors, and is also quick to grasp the student’s strong and weak points.
At some point (ca. 2017-20) he was lightly involved with the Open Study Room, if I (bugcat) recall correctly, providing some lessons to league winners and medal placers, payment for which was provided by OSR. At that point, at least, he still preferred to review on KGS rather than OGS.
He sat with Antti on the anticheating committee of the third Corona Cup, held in 2021, and perhaps in the other editions as well.
Some of his lectures can be found on Youtube.
He's an awesome guy and a great teacher. :D -- full body of the first version
Interview from 2012: http://gosensations.com/?id=2&server_id=1&new_id=1343