Replacing all vehicles can be an annoying process, as I need to send all vehicles to the depot, sell all of them, buy the new one, and perhaps the most annoying part, is to release them from the depot again with precise timing. Simutrans Extended may help, but it's way too complicated and doesn't support some of the paksets. What can I do?
Add all convois to a line (there is a function for that, if they have individual schedules), then go to an depot and add the replacement vehicles. Click on withdraw all in the line and start the replaced convois.
Nevertheless, Simutrans was made in a way that actual replacing is a seldom event.