Does anyone know if upgrading firmware will modify the default root password?

[bjmc]

I'm considering upgrading to the latest firmware from the Zyxel FTP site, but I don't want to lose root access to my router. According to this the key/password derivation functions are based on hardcoded parameters from an individual unit, like its serial numbers. Presumably those parameters will stay constant, even if the firmware changes, but I suppose it's possible they've modified the password-derivation algorithm in the new version.

Does anyone know if upgrading the firmware on Zyxel units results in a different default root password?

[ThomasRinsma]

In my experience over the last year with several updates, the passwords were not changed. It is indeed also true that there are mechanisms to derive the different passwords, but I think they are not used anymore (at least in my case). At least not for admin and supervisor: those seem to be set dynamically by the ISP.

Either way, there is of course nothing preventing Zyxel or your ISP from modifying any of the passwords, or the related code.