Você não deveria. O Elementary OS Luna é construído no Ubuntu 12.04 LTS, e é provável que seja quebrado instalando novos pacotes. Não há garantia de que eles irão funcionar.
Quanto a estar desatualizado, isso não está correto. É verdade que existem pacotes mais novos, mas os mais antigos ainda estão sendo mantidos, o que significa que as correções estão sendo produzidas ativamente e os patches de segurança são aplicados a todos os pacotes gerenciados pela Canonical. Afinal, este é o significado da expressão LTS = Long Term Support.
Se você ainda quiser tentar atualizar o seu sistema depois disso, sabendo muito bem que você pode quebrá-lo mal, você pode tentar adicionar as seguintes linhas ao arquivo /etc/apt/sources.list, ajustando o local de acordo com o seu necessidades:
deb http://us.archive.ubuntu.com/ubuntu/ saucy main restricted
deb-src http://us.archive.ubuntu.com/ubuntu/ saucy main restricted
deb http://us.archive.ubuntu.com/ubuntu/ saucy-updates main restricted
deb-src http://us.archive.ubuntu.com/ubuntu/ saucy-updates main restricted
deb http://us.archive.ubuntu.com/ubuntu/ saucy universe
deb-src http://us.archive.ubuntu.com/ubuntu/ saucy universe
deb http://us.archive.ubuntu.com/ubuntu/ saucy-updates universe
deb-src http://us.archive.ubuntu.com/ubuntu/ saucy-updates universe
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://us.archive.ubuntu.com/ubuntu/ saucy multiverse
deb-src http://us.archive.ubuntu.com/ubuntu/ saucy multiverse
deb http://us.archive.ubuntu.com/ubuntu/ saucy-updates multiverse
deb-src http://us.archive.ubuntu.com/ubuntu/ saucy-updates multiverse
## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://us.archive.ubuntu.com/ubuntu/ saucy-backports main restricted universe multiverse
deb-src http://us.archive.ubuntu.com/ubuntu/ saucy-backports main restricted universe multiverse
deb http://security.ubuntu.com/ubuntu/ saucy-security main restricted
deb-src http://security.ubuntu.com/ubuntu/ saucy-security main restricted
deb http://security.ubuntu.com/ubuntu/ saucy-security universe
deb-src http://security.ubuntu.com/ubuntu/ saucy-security universe
deb http://security.ubuntu.com/ubuntu/ saucy-security multiverse
deb-src http://security.ubuntu.com/ubuntu/ saucy-security multiverse
## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
deb http://archive.canonical.com/ubuntu/ saucy partner
deb-src http://archive.canonical.com/ubuntu/ saucy partner
Mas lembre-se: você foi avisado .