I think the game automatically buy you a new vehicle of the same model if you forget to change it past a certain time (I've seen some of my vehicles 2 years older than usual still running) but if you talk about upgrading an old vehicle with a new model and still use the same route, here how to:
Open your depot window and your new vehicles window. Sell the vehicle and immediatly buy a new one. You'll have a small delay for this but make sure you don't wait too long. When done correctly the old route will already be integrated in the new model memory just like the scrapped model. Just make sure it does run at the right target (especially for trains) after the exchange and you are done.
k:
Also, I tend to make my vehicles get serviced sooner than the suggested default, but I judge this about the % reliability of the vehicle. Anyways, the service time will be default for the new model so make sure to change it back if, like me, you don't like it.