Bit Twiddler said:
"Say, for example, part of the fuselage needed changing and L3 swapped it with a part off the rest of the RJ fleet which had an incomplete history and completely different flying hours, then what happens."
This "exchange process" is actually quite common throughout the world's aviation regulations. However, it is (in 99% of all the requirements I've dealt with) based upon confirmed knowledge of the status of the replacement part prior to installation.
This works well where you have properly logged and documented parts removed, preserved and stored, from airframes withdrawn from service for economic reasons or even some that have been designated BER.
To use parts that have no known history or documented record of usage is just playing with fire. They may have the look and feel of OEM parts but they are no better than Bogus Parts.