Caveat: Hypothetical vowel shifts, just because…

It’s hard for me not to think about linguistics, sometimes.
As I was walking to work the other day, I had this strange thought: I wondered if, historically, the Korean language ever under went a major vowel shift – of the same sort that is somewhat well-known with respect to English. I think I was set off on this train of thought by noticing that, although Korean is for the most part rather punctillious about its many vowels and their correct articulation, there is definitely a degree of movement taking place in the Seoul dialect, which is noticeable in, for example, my students’ “playful” spellings of phrases like 안녕 [annyeong = “hi”] as 안뇽 [annyong] in their notes or text messages to me. The sound 어 /ə/ is definitely moving backward in casual speech, toward 오/o/.
Further evidence lies in the fact that although the consonantal system of the hangeul writing system is quite reliably systematic, the vowel system has always struck me is a bit “off” – it doesn’t seem quite as systematic as it could have been (i.e. why are some vowels written under and some vowels written to the right, and where’s the original logic behind the design, given there is consensus that it was designed?). Of course, it is well known that the “offness” of English spelling is, itself, a consequence of that famous vowel shift. So maybe the cause of the “offness” in the Korean vowel system lies in the same sort of phenomenon.
So when I got to work, I typed “vowel shift korean” into my google search-o-matic, and sure enough, my thought wasn’t a novel one. Indeed, there is a still not entirely settled but well-defended hypothesis that Korean underwent a major vowel shift, roughly during the same historical period as the English one. Mentions of it in English can be found online (mostly in googlebooks, rather than on blogs or webpages).
Thus, it is mentioned (and supported) in Samual Martin’s (he of the giant Korean Reference grammar with its despised Yale-ification) book, Consonant Lenition in Korean and the Macro-Altaic Question, somewhat browseable on googlebooks. The best explanation of the idea that I found, with some charts of the actual shift, is in a PDF by Oh Sang-suk. Meanwhile, I found a refutation of it in a PDF by Young-Key Kim-Renaud.
Personally, I find the idea appealing, and the arguments in favor of it that I ran across struck me as compelling. I am not an expert, however – so who knows?
[daily log: walking, 6km]