I got an email from Vanguard about this a few days ago. It was phrased much more forcefully, I thought, than prior e-mails, or at least seemed so to me. It said in part:
So OK, this time I got fooled and went ahead and did it about an hour ago. In the process they mentioned that all sales would be using the FIFO cost basis, not averaged, and I have been using the averaged method all these years as computed by Vanguard and doubt I still have the records needed for FIFO.
They said that if we don't like that we can always change it back again. Oh goodie. Who knows what else I will need to do just to maintain things the way they were. This sounds like a real mess to me.
BTW, they mentioned that for a day or so during the transition it might be harder to access one's account. Also they give you a half dozen PDF's explaining things and the website almost timed me out because I was trying to at least skim over them briefly.