Can nls_characterset be changed after the oracle server being installed?

I took the default install of Oracle server and the nls_characterset is set to 'WE8ISO8859P1'. Now I need to make my Oracle server to support Unicode, so how can I change the nls_characterset to 'NTF8'? I heard it can only be changed when install the oracle server.

Also, do all the instances on an Oracle server have to use the same nls_characterset? Can we assign different ones for different instance?

Thanks a lot!