Ah, you're correct. I was thinking of the shift that occurred in the transition from Old English to Middle English, which was put in motion by the Norman Conquest. The GVS is actually the major pronunciation change between Middle English and Mod.E. Had to dig up my Pyles and Algeo and check, as I always get mixed up. Interestingly enough, P&A state that aspects of GVS were still occurring into the 1800s.
You are right, though, the vocabulary changed after the Conquest, with the rise of Anglo-Norman. Although the general populace never spoke French, loan words crept into usage. P&A label the Middle English period as 1100-1500, and the difference between Old English and Middle English is considerably greater than the difference between Middle English and Modern English. In a HP Universe context, any documents from the founding would probably be in Latin, with a small chance of OE. So, as long as JKR sticks with Latin for her really old stuff, she should be OK. What we shouldn't ever see, though, is fake ME for OE.
Re: blah blah Up Slytherin
You are right, though, the vocabulary changed after the Conquest, with the rise of Anglo-Norman. Although the general populace never spoke French, loan words crept into usage. P&A label the Middle English period as 1100-1500, and the difference between Old English and Middle English is considerably greater than the difference between Middle English and Modern English. In a HP Universe context, any documents from the founding would probably be in Latin, with a small chance of OE. So, as long as JKR sticks with Latin for her really old stuff, she should be OK. What we shouldn't ever see, though, is fake ME for OE.