The pronunciation of "was" has changed since the Seventeenth Century. When and why did it change"?

In Chaucer and John Donne "was" is pronounced like "wahss" but nowadays we say "woz". When did the change occur? and why?

Perhaps my Google-fu is lacking, but I am finding it exceedingly hard to find any actual, relevant sources for this; but I’ll answer nonetheless.

Some time during the 17th century, a sound change occurred in English that rounded the short vowel /a/ when it directly follows a labialised sound (/w/ or /kʷ/ [written ‹qu›]).

In general, the change meant that /a/ (probably pronounced [ɑ] at the time) became [ɒ] (or [ɔ], as is most common in American English today—I don’t think anyone knows what the exact quality of the vowel was back then) after labialised sounds, except if followed by a velar (/k/, /ɡ/, /ŋ/) or bilabial (/p/, /b/, /m/), in which case it was fronted to [æ] instead.

In other words (with some later changes, like the automatic lengthening of /a/ before voiced plosives and the widespread, though not ubiquitous, deaspiration of /ʍ/ to /w/):

ɑ > æ   / w kʷ ⸏ k ɡ ŋ p b m

whack [ʍɑk] > [ʍæk] (> [wæk])
quag [kʷɑɡ] > [kʷæɡ] (> [kʷæːɡ])
quank [kʷɑŋk] > [kʷæŋk]

ɑ > ɒ   / w kʷ ⸏ [elsewhere]

was [wɑs] > [wɒs] (> [wɒz ~ wəz])
want [wɑnt] > [wɒnt]
what [ʍɑt] > [ʍɒt] (> [wɒt])
wad [wɑd] > [wɒd] (> [wɒːd])
wap [wɑp] > [wɒp] *
wabble [ˈwɑb(ː)l̩] > [ˈwɒbl̩] (now written ‹wobble›)
war [wɑɹ] > [wɒɹ] (> [wɒːɹ, wɒː])

* Not to be confused with WAP (Wireless Application Protocol), which is obviously a much newer one—and one that proves that this sound change is no longer productive, since it is pronounced [wæp], not [wɒp].