Wirelessly posted (BlackBerry8530/5.0.0.601 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/417)
"Also, if you get a J reward on BA from SA to NA, you will travel J only to your NA entry point (MIA, NYC); if you need to go to ORD or DTW, etc, it will be in Y as there is only Y or F and BA will give you only Y for the domestic segment."
There is an important exception to this rule: if the international segment is longer than the longest domestic segment on the award itinerary, BA will then permit you to fly in two-class F on the domestic segment(s).