Next Train Stock

To actually answer the question, you do not need to alter 1A05 (even if that would be sensible!), as the train will run.

1A05 will have the same properties of the inbound 5A05, much as you would expect if you substituted another train for 5A05 when playing due to some disruption, and magically changing train length wouldn't work well as you suggested.

The most obvious issue is that it doesn't seem possible to easily work out what stock a train is actually made of if not the timetabled stock. The timetable for 1A05 will still show it as 11-car, and the F2 "Description" column simply copies the description of the timetabled stock. The "Length" and "Power" columns do at least represent the actual train.

I sometimes wonder where people get their ideas from, traintypes have never been auto generated, Splodge you will need to alter 1A05's traintype

If 1A05 has been duplicated from 1A03 it will still be a 9 car as the train has been duplicated and will give a length error. Duplicating a schedule, as in a carbon copy keeps all the information of the previously provided schedule, in this case 1A03, which you then alter to 1A05 and redo the times if necessary.

In F2 the stock type is read from the Train Category on Train Characteristics tab in the editor and is not generated from any previous working. Every Schedule must have its own traintype.