You can't just swap the engines on a 747 or any other mainstream jet. They need different pylons, wiring, hydraulic connectors and a host of other things.
Recognising this problem and how it can affect the resale value, Boeing is designing the 787 in such a way that it will be possible to swap power plants. This is in part only possible due to the fact that most systems that were hydraulic will be electrically actuated on the dreamliner. Is this a good thing - well.......... if god/allah/budha forbid both engines were to go, you would be left with a ram air turbine between you and an uncontrolled and unscheduled termination to your flight!