This is unclear. With your N95, changing the product code is a definite warranty voider, and always has been. The N900 doesn't need a product code change though, all you're doing is installing a new firmware (provided by Nokia) using the flasher tool (provided by Nokia) onto an N900 (provided by Nokia). I've spoken to one person at Nokia Care who said that changing the firmware would void the warranty, but logic (along with the fact that the tablets-dev page specifically states you're authorised to use the firmware versions it presents once you enter your IMEI, and this includes the Global variant) suggest otherwise.