I've ALREADY bought our tickets for our Nov/Dec trip from Boston.
I paid $181 and thought that was fine cause I like to have specific days, times, and non-stop and that's Thanksgiving weekend. It's fluctuating between $199, $209, $219 so I'm happy.
Anyway, my tickets say the plane will be a 757.
I'm not going worry about this. I'll check frequently and if they make changes I'll be calling to fix it. I check almost daily and I read here.
I've been lucky so far, in all our trips down there and buying my tickets so early, not to have major changes, just minor ones; by 1/2 hour or so or the flight # changes.