It is not possible to change this value in the GUI, but can be done within the raw database - Use with care!
However this is more of a procedure issue, where the previous incarnation of the device could be retired by using the Unmanage option and the new device added.
I would be interested to know if this value could be updated as part of the Rediscovery process.
It's not possible to change MachineType manually. This is stored according SysObjectID and other properties. You even can't change it directly in database, because it will be overwritten with next rediscovery poll.
Are you saying that rediscovery poll is not able to update MachineType correctly on all your nodes? Do they have correct SysObjectID on Node Detail resources at least or it's also outdated?
Not sure how to see the SysObjectID? I'm looking at Node Details on the web interface and don't see it. Are you saying look in the database?
I have done a rediscover and it doesn't update the machine type. In case it helps we are running NPM 10.2.2 and NCM 7.0.