In the case of 'Heritage' replacement bodyshells, there's no identity switching going on. A replacement part is being supplied by the original manufacturer - or an agent thereof - and the chassis number is usually stamped on a plate which is affixed to that replacement bodyshell. Key point is that there is no second or third party - already existing donor with its own identity - involved in the equation. This was always the case with the classic 'body in white' replacement monocoque/bodyshell from a manufacturer.
With the Nissan S30-series, the chassis number is engraved on the firewall at birth - effectively making the bodyshell/unibody unit 'The Car'. Part Number One. Supported by its engine bay-affixed chassis tag (stamped with the chassis number) and - in the case of North American market versions - the door jamb tag and dash tag, this forms the unit's identity. Theoretically, it's not a moveable feast. You can chop and change a car around, strip it down to a 'shell, hybridise, bastardise and mess around as much as you want (subject to inspection...) but you can't legally change its identity without full disclosure to the licensing authorities and acceptance of their choice for its destiny (including their VIN and a 'Q' plate, if they see fit). That's the situation in the UK as I understand it.
There are no replacement bodyshells available from Nissan anyway, so the 'Heritage 'shell' situation as far as it applies to 'us' is a straw man.