Crumbs!! This is getting a bit like Trigger's broom. At what point does this stop? How can you be certain that you'd seen the same loco right down to the last bolt and washer? What if the screenwash bottle has been refilled? What if the driver had forgotten his flask?
I appreciate that everyone has their own ideas and rules, but I will just offer this from the world of aviation.
When I was still a callow youth I used to know a chap who served in the RAF at Wyton in Cambridgeshire where he was involved with some of the last operational Canberras at around the time of their withdrawal. According to him, an individual jet engine was comprised solely of an identification plate. It didn't matter what else was bolted to that plate, it was the plate and the plate alone that identified that engine. If you possessed the plate then you possessed the engine and any engine without a plate was not an engine no matter how complete it was. Therefore an engine could in fact be completely materially different, but it was still the same engine provided that it had the same identification plate.
**EDIT**
On a slightly more pertinent note, is it not the case that sometimes a preserved loco will carry the identity of a long-scrapped classmate, or indeed a totally fictitious one? In this instance, what number do you record?