One of the most important, yet least documented phenomenons of Central Asian history was the "replacement" of Iranian-speaking peoples by Turks between the 4th and 5th centuries.

At least back in the 2nd century and before, most of what is now Turkestan, Sinkiang, and Western Mongolia were populated by Iranian-speaking nomads. By the 6th century, however, most of this region was dominated by Turkic-speaking peoples.

Has anyone made a study as to when and how this transformation took place?

Was it a result of the Xiong-nu's western migration or the expansion of the Gokturk empire?

Did the Iranian peoples simply become absorbed by turks, or were they pushed further westwards?

"Eastern Turkistan and Western Mongolia was populated by Turkic speaking peoples way before the 2nd Century, the Xiongnu held sway in that region for centuries before and their ancestors before them."

total fictional! as the the evidences available testimony that these areas was surely populated by the Iranics and the pan-turkists claim that there were yet altaic groups back in the days even before the indo-iranians inhabit the region, which is actually a theory lacking evidences.

"Eastern Turkistan and Western Mongolia was populated by Turkic speaking peoples way before the 2nd Century, the Xiongnu held sway in that region for centuries before and their ancestors before them."

Yes. I say again fictional. Before the second century BC even considering the obscurity in sources no evidence the land of ET and the whole Mongolia belonged to them and their ancestors.

The Xiongnu was mentioned in chinese sources somewhere north of the great wall and their first appearance was probably around 3rd century in these sources. And they migrated more westward likely pursuing the Yuezhi. that's not related I said the theory backed by Turkist scholars tries to expand it even before the 3rd century and alike before the indo-iranian recorded presence in eastern turkestan or some parts of mongolia, the area belonged to the proto-altaics, this lacking evidences to a great degree. 

Cite the chinese sources you mean (10-8 BC?! it's a new on my sundae). in most of the books refering to the chinese documents including Xiongu's it was as early as about 3rd century BC. Ok can't wait to see the  so-called AGREED UPON sources.

The Yuezhi settled, after their migration in Tocharia nut yeah the language is believed to be a Centum rather than Satem.

That would be intersting 10- 8 Centuries BC. I know that they were described Xiongnu. but they most of the times mentioned north of the Great wall! And the time they mentioned in xinjiang and Eastern Turkestan is about 3rd century BC. 

"In fact, Chinese records describe the history of Xiongnu much more detailed than Greeks describe Scythians. And as I said, the location of Xiongnu and proto-Xiongnu tribes is accepted by the mainstream of historians. If you're saying it's fictional, you actually are agitating against the facts that are more or less agreed upon."

Of course no denying of it also chinese gave account of Iranics and Yuechi probably well enough comparing to Greeks.

The Yuechi not being Iranic at all show the Indo-european presence in north of Great Wall (where today is considered eastern Mongolia) as well.

That's correct, but in which period if it's a discussion about who were the earliest to Inhabit the lands even further east than Xinjiang. 

You were telling me about the 10-9 centuries BC records of the Xiong Nu, I'd like to know the exact source of that in chinese annals or records?! 

I think the whole argument started because you didn't agree that Proto-Turks were in Xinjiang before 2d century BC, while they definitely already were there at that time.

In ancient times, Saka (East Iranian) languages were spoken in Afghanistan, Sistan and Central Asia. Now Pashto and Pamiri languages are still spoken.