Once when I was in CE on business and my GF was in ET, on boarding I asked the FA checking BPs if it was OK for us to switch. She nodded yes, and we duly swapped BPs and I went to the back of the plane. I suspect that asking at check-in or boarding might turn out to be making a mountain out of a molehill; once on board (even before take-off) it is more within the FAs control.