Just to wrap up on this, the very nice agent at the First Class line at the check-in counter at BWI did this with ease. So now I'm going BWI-MIA-SFO instead of BWI-JFK-SFO.
That's a good thing as I get 500 more miles, and avoid the T9-T8 transfer (JFK-SFO would've been from the evil Gate 4x). I'd prefer not to have to declare my fluids all over again.
Unfortunately, it looks like BWI-MIA is running 10+ minutes late - which puts my transfer at 30minutes or less at MIA. I have a bad feeling about this - would hate to be stuck at MIA overnight!