Assuming you're asking about that, NGUI has no hardware/system based authentication so you will be able to work with it normally no matter the change.
However, the license for Unity itself is indeed attached to your PC, so I'd recommend reading up on deactivating it before you change the system.
Once you set up Unity on the new PC, all that would be necessary is to enter the Asset Store from your properly activated Editor and reimport the NGUI package from there (or just to reopen your project).