Hi Richard,
The G4 Plug-In range is a direct replacement for your factory computer, this means it will not be compatible with anything but a similar model car.
If you were to get a Wire-In ECU (G4 Xtreme http://www.linkecu.com/products/engine-management-ecus/g4-xtreme or G4 Storm http://www.linkecu.com/products/engine-management-ecus/g4-storm) then you would be able to buy a new loom when you change cars and wire-in to your next car.
This leaves you with the issue of being able to sell your old car as a running vehicle. If you were to purchase the Wire-In with the optional 400mm 'short' loom, then you could splice into your factory harness, leaving your factory header in place to swap back at a later point. Just make sure you don't plug both computers in at once!
-Cameron