It’s likely they have altered the booking as part of pricing the change. However, they may not have reverted the booking back to the original, which is why it is currently showing as F. I would not expect this to stick. You may want to phone if you are keen to ensure you get your choice of J seats. However, you could always leave it and see what happens.

I have the same for a flight in June, so just leaving it as happy to take a chance. I suspect it will revert back to Y at some point.