Hm, well... then the unit would look like a bridge, but nobody would actually be able to cross it, so the entire 'bridge' function is lost if you just replace the model.
What exactly are you trying to do so you really need a unit?
Because perhaps you could make a dummy unit and link a doodad-bridge with it (triggered, of course). The doodad-bridge would always be at the unit's location, so it appears as if you can move the bridge, but you're actually using a dummy unit that is linked with a bridge.