Looking at the records of Clun Castle 7029 it can be seen that it had 6 different boilers between 1950 and 1962, and one particular boiler had been on as many different locos. It had 9 different tenders, 4 Hawksworth, and 5 Collett, and again one of those tenders was associated with 9 different Castles.
As far as the average trainspotter in my youth was concerned, what was of importance was the number on the cabside (a brass plate as per GW locos or simply "painted on" in most other cases) or the front number plate on the boiler in the case of steam locos. It was only when neither was available, as in the case of many GW locos whose plates had all been removed, that trying to get a number from other remaining parts such as coupling rods was resorted to. We did sometimes find the number on the coupling rods on the other side of the loco didn't match when we took the trouble to double check we had got the number correct, or compared notes with our mates!
In general the number of a loco going through works will not have changed but the loco frame, boiler and everything else could potentially have been replaced, or swapped with other locos over the years. There is nothing to say, of course, that the number plate on the firebox were all originals as a few may have had to be replaced due to damage or other loss (theft?). So it is, of course, in preservation where many plates are duplicates as the original is lost/destroyed/stolen or owned by someone unwilling to part with it. With that possibility virtually nothing is left of the original, beyond it's record card if that exists.
The fact that the number/plate was about the only thing which didn't change possibly explains why some smokebox plates (and GW cabside plates, though of course there were two of those per loco!) fetch such a high price - name plates even more so - as they are more or less guaranteed to be original (though some may sadly be fakes!). I gather that firecut diesel loco cabside numbers are of similar interest. Can't say that I've seen one from a steam loco cab.
As spotter we didn't even consider the chance that two or more locos of the same class in works at the same time might have accidentally (or deliberately e.g. 46152/46100) acquired a change of identity after a repaint where the cabside numbers were temporarily lost and smokebox numbers exchanged. Apprentices are renowed for their tricks!
Personally I stopped adding further steam locos numbers to my "combined volume" in August 1968. What was underlined up that point is, for me, my definitive list of what I've seen. It's of no real interest to anyone else and, although it may not be 100% accurate, it is one of my treasured possessions. Even 60008 (the A4, not the diesel!) which I saw for the first time when it came back from the USA won't get underlined! I keep a separate list of the numbers seen on preserved locos.
The changing of numbers (e.g. another preserved loco appearing as 44984 for an anniversary of the last day of the GC in 2016), or for that matter such as D123 carrying the name plates of D163, I find irritating but won't lose sleep over it! At least we have preserved members of many classes and whether the number they are were allocated when built is the one they normally carry isn't that important to me personally. Just a big thank you to all those actively involved in preservation. Without you we'd have nothing.
The one plus side of new builds (45551, 60163 and others) is that their identity will never be in doubt! Personally I would have allocated a new number to such as 1014 County of Glamorgan rather than taking the identity of one scrapped but am thankful that a "new" County Class loco is being created so it is not meant as a criticism.
Finally - in case anyone has read this far - if anyone wants to cop a Standard Class 2-6-4T I have what I believe to be an original firebox number plate

(Pity I have nothing else of the original but, there again, if I'd had the money to preserve the loco to which it was attached immediately prior to its demise there is nothing to say I'd have much of the original beyond my number plate).