I think you'll be surprised to how much influence OTL Korean culture has received from the Urallic tribes
Did I say the contrary? What I'm saying is that I don't think an PIE presence that was certainly IOTL much mixed on the northern steppes isn't going to be dominant for the sake of being more present in its eastern parts.
I think a Korean culture almost completely dominated by an Indo-European language is quite possible.
Why? For all we know PIE influence that might have existed is quite restricted to loanwords and material features.
Wusun peoples, which might be the last direct elements of the Afanasevo culture in northern steppe, were most probably mixed with Proto-Mongols and Proto-Turks. And that was in the western part of the Mongloic/Turkic ensemble.
Genetics studies seems to point that there, you had already a significant inter-breeding between PIE/PIEzed peoples and Mongoloids peoples. More you'd go east, more mixing you'd get.
So let's now assume some traits of the European steppe peoples - frequent use of ochre, animal remains in graves, the PIE language, and so forth - becomes transferred to Korean culture by 2000 BCE.
It borders chronological impossibility. By 2000BCE, the decline of Afanasevo culture eventually brought on non PIE influences more importantly in the region (possibly due to demographical booming, itself due to techological changes brought during Afanasevo), while distinctivly PIEzed cultures as Andronovo were more western and southern leanings.
While I could see a PIE push in the north-eastern steppe it will have to blend with Altaic and precisely Proto-Mongol and Proto-Tungusic.
At this point, arguing of a PIE Korea is a nonsense : PIE was already divided into distinct cultures and languages since
thousands of years in the IInd millenium.
It is obvious that we'll be then talking of a culture more or less close to Sycthians (themselves the
less further culture compared to the IVth millenium situation, but really far objectively nevertheless).
Let's say this
Amur Culture (gonna simplify things) exist, it would be an important blend of different populations, on which Europoids (which is to be distinguished from IE/IEzed peoples) would be only a part that would be less important with the development (demographical or else) of Altaic peoples.
With contacts with other IE/IEzed cultures as Andronovo would be further, the IE presence in Amur would be an isolate, which is never faring much well on the long run.
Giving the lifestyle of steppe peoples, furthermore, I'd expect a focus on Korea being more likely in the Formative Mumun or later, as for exchange possibilities and eventual takeover.
Something akin to the interaction between Trypilla and Kurgan cultures, if you prefer.
Long story short : a cultural IE element that could be identifiable, archeologically, and influential (as it may have happened in China).
But dominating? Giving how much it didn't went this way IOTL, that's streching things a lot.
As for the changes...
I'd expect the appearance of a possibly more hierarchized society, would it be only as a reaction on steppe people presence : maybe a division of Korea into two roughly different cultures? As in :
- A steppe Indo-European/Proto-Mongoloid ensemble, with other Altaic substrate, a continuation of *Amur Culture
- A southern Korean culture, maybe more tied up to Liaoning culture ITTL, possibly swalloed up by its northern neighbours, but forming a huge substrate as well in the best of case.
Basically, I'd expect a different Altaic Korean culture, maybe significantly more tied up to its steppe neighbours while still inheriting the same structure than IOTL Korean development. Think Kithans on steroids.
