Taking the TOEIC is almost a rite of passage out of college and into corporate society in ROK. Unifying the Koreas will expand this.
My ROK soldiers at I Corps and KATUSAs at 2nd Division usually spoke pretty decent English, especially the ROKs at I Corps as they were sons of Chaebol, senior military men and high government officials, hand-picked to work in our headquarters.
I think that if that happens, it might happen in a few generations. I say that because everybody there now is completely brainwashed into thinking the Kim family is descended by whatever god they're forced to believe in. It will take a long time for them to completely immerse themselves into the world-at-large.
I'm throwing a guess out there that many will refuse to assimilate, and would sooner attack an American or South Korean than have friendly conversation. But, that's just my opinion.