Fedex will almost certainly choose its engine based on price and politics. Past engines will almost certainly play no role. As a matter of fact, probably the sole reason that FDX doesn't operate any Rolls Royce engines is because our fleet types never incorporated them in the first place. Its pretty obvious to me that FDX doesn't subscribe to the conventional wisdom of engine commonality....as a matter of fact they must subscribe in a theory that's diametrically opposed to engine commonality. But I'll venture to guess that they'll go RR.
You're only as good as your last departure.