I’ll amend my original idea to say there should be two buttons that are on every vehicle window (and maybe even popup with the “old equipment” notifications):
Replace and Sell
Replace: Would give you the same interface for buying that type of vehicle from the appropriate depot. Just click the vehicle you want and the old one drives to a depot to sell itself, then the replacement is bought and applied to that line. That’s only 2 clicks – 1 to click the button, 1 to choose the replacement.
Sell: Drive the vehicle to the nearest depot and sells it. Same functionality as the button that already exists for the line as a whole. Why the same button isn’t available “per vehicle” is beyond me.
That would make it much easier to replace vehicles, and gives the player more control than a fully automated solution, yet is no where near as tedious. And if you don’t want to replace them all (as has been pointed out), you can easily choose which to replace and which to sell.