changing domain name

[Niels Rasmussen <nielsrasmus@gmail.com>]

Hi list

I'm running testing for amd64.

niels@debian:~$ hostname -a
debian  debian  debian.opasia.dk

I would like to change this to:
debian.nirvana

How can I do this with a simple command, so this would reflect the
whole system ?

When installing testing I was connected via a wlan router (D-link
DI-524), the installer suggested opasia.dk, my ISP provider,
which I (in my ignorance) accepted.

Is this possible without cutting my internet connection ?

Thanks for your time.

-- 
/Niels
Registred Linux user #133791