I think it's as simple as if you're qualified to fly as a CA, you're qualified to fly as an FO - which in theory is less responsibilty. I don't see what the big mystery is. At XJT, if a CA is assigned by the company to fly as an FO, he/she still get CA pay. If a CA were to pick-up open-time as an FO, they'd be paid as an FO, but this situation still needs approval by the company. Back when we had T-props, there weren't any requirements that dictated a t-prop pilot upgrading to ERJ CA fly in the right seat. In fact, in the mid-90's, it wasn't uncommon for a Brazilia new-hire to be awarded Brazilia CA while in training and within 12-18 months to be in ERJ CA training.