I don't think that the complimentary upgrade for EXP gets you rebooked in first during IRROPS by policy (though YMMV and someone please correct me if I am wrong). You probably should have been added to the upgrade list though when you were rebooked into coach.
With that, I am not sure what pointing out an empty first class seat would do except further annoy you. Most (all?) of the time, if an agent doesn't give you a go-ahead, it isn't going to happen.