Just to add a data point, we have just completed a Chase 241 BOS-LHR-BOS, out in F, back in J (would liked to have done it the other way round but there was no availability).
We returned to BOS on the 203 on Friday 8th September. The 229 to BWI had been cancelled and at least some of its pax were rebooked onto our flight which was about 45 mins late departing as a result. Up until we boarded and got green beeps at the gate, I was dreading us being downgraded, especially as I'd got us CW centre bassinet seats at T-24 OLCI so we could have direct aisle access. But in fact there were no problems and I'd say the lower deck CW was about 1/4 to 1/3 empty.