The best way I’ve found is:
- pause the game
- open the line dialog and click the $ icon to sell all vehicles on the line
- buy the replacement vehicles from a depot and set them all to the line
This is the only way that I’ve found to do this that doesn’t seem to affect customers using the line through the frequency changing due to no vehicles being assigned while the sim is running. Obviously it’s no good if you only want to replace some of the vehicles too.