Most carriers will auto rebook onto the most convenient routing, but they cannot determine the individual passenger's preference. Thus, the first thing to do is to look at alternative routings and have those ready for when you call. By providing specific flight numbers, you make this easier.

You may also find that the scheduling is such that J is the better alternative and you then have to make a decision.

For an involuntary reroute, you are due a refund of the fare difference between F and J for the downgraded segment. Make certain that you deal with this as a refund, not compensation as the two are entirely different.