Admin,
I think it's kind of irrisponsible to mention debranding and/or changing product codes and not mention the issue of voiding your warranty with Nokia and/or your network (many have to get replaced via network if on a contract - like me on Orange). Also many netowrks have actually started adding clauses in contract supplied handsets mentioning unauthorised debranding, feature removal, or removing SIM locks. Upgrading seems fine.
Nokia service points and networks know simply by looking at the menus, let alone the software version & variant that a phone has been debranded.
Many users decide to take the risk to get debranded. Personally knowing the issues with many UK networks faced by users of other devices (regular user at Nokia forums under another user ID) there are many users stuck with a £500 device which no one will repair because of debranding and/or product code changing.
This covers it (hmmm familiar user name ;) )
Firmware upgrades. (Sticky) - Hardware, Codes, and Operator Issues - Nokia Support Discussions