I would call YouFirst and see what they have to say and ask what your options are. You don’t have to take any option they give you today, but at least you will know where you stand. I don’t know if they will allow you to travel ok a different date given there is availability on the day on BA and alternative options with AA or a combination, but in case it helps there is availability and good seats either side.
I suspect if you volunteered for J now,,you’d just get the difference in Avios, if you waited until the downgraded you on the flight that would then be involuntary and the EC261/04 mat apply- although it can be complicated and a challenge to get compensation on a reward booking (persistence & id suspect you’d get it but be prepared to fight for it in case).
18 March
BA 295
LHR 12:10p → ORD 4:05p
F4 A3 J9 C9 D9 R9 I9 W9 E9 T9 Y9 B9 H9 K9 M9 L9 V9 S9 N9 Q9 O9 G
BA 297
LHR 3:35p → ORD 7:30p
F9 A8 J9 C9 D9 R9 I9 W9 E9 T9 Y9 B9 H9 K9 M9 L9 V9 S9 N9 Q9 O9 G
20 March
BA 295
LHR 12:05p → ORD 4:00p
F9 A8 J9 C9 D9 R9 I9 W9 E5 T0 Y9 B7 H4 K2 M0 L0 V0 S0 N0 Q0 O0 G
BA 297
LHR 3:35p → ORD 7:30p
F9 A6 J9 C9 D9 R9 I9 W9 E9 T9 Y9 B9 H9 K9 M9 L6 V3 S0 N0 Q0 O0 G