Airlines change their liveries from time to time, but why should this bother us? As I mentioned in a previous post, some collections of aircraft are published to shared paths and some are not. If an author uses a path that several aircraft are published to then X-Plane will pick a random aircraft each time the scenery is loaded. For example, most b747-400s are published to /objects/aircraft/jets/heavy/b747-400.obj but some one-off liveries (Air Force One, for example) are not included in this, so you will never get a randomly placed Air Force One – the scenery designer has to explicitly choose to place that aircraft for it to appear.
One specific category of aircraft that is excluded from random placement are those aircraft that are in historical liveries. Historical liveries are present in the library because some authors may want to use them under specific circumstances, however when randomising we don’t really want old liveries appearing, as it would look strange in a modern airport.
So when does a livery become historical? Well, you may say, surely it becomes historical when the airline starts painting a new one on its aircraft? Well, I say, but what if no-one has contributed the new livery to the library yet? Should all British Airways aircraft simply stop being placed randomly the moment BA change their colours, until some generous person contributes a new livery?
The answer is no. The old livery remains active and current until a new one is submitted. At that point the old livery is moved into a sub-path (e.g. /objects/aircraft/jets/heavy/b747-400/british_airways.obj would move to /objects/aircraft/jets/heavy/b747-400/british_airways/historical/1984-1997.obj and the new livery would replace the old aircraft).