An overview is shown in the image.

In the first place, the current vehicle is displayed on the class setting dialog. They are not vehicles to be replaced.
Therefore, the modifiable lists do not match. Only the one that happens to match will be changed.
For example, open demo.sve and add a 2-car LNER Tyneside EMU without a mail vehicle to Train commuter Chrisminster.
Then try to add a mail vehicle to that vehicle with Replace.
You cannot change the mail class. Because there is no combo box for the mail class.
Another example, replace it from 2 to 4 cars. Please change the class at that time. Only the previous two cars classes will be changed.
Note that it has only changed because the previous two types match. If they do not match, none of the classes will change.
Also, again, as someone pointed out, the capacity displayed under change prices is also incorrect. Because it doesn't count more vehicles than the one before the change.