Sources list trouble?

[geoaraujo]

Hello fellows!
Is there a way (or is it necessary or even desirable) to remove sources list entries from previous installs? I'm running 13.04 now, but I've been upgrading since 11.04.
I'm asking this because now I'm receiving this message when sudo apt-get upgrade && sudo apt-get dist-upgrade:
[HTML]W: Falhou ao buscar http://br.archive.ubuntu.com/ubuntu/dists/natty-backports/main/source/Sources 404 Not Found [IP: 200.236.31.4 80]

W: Falhou ao buscar http://br.archive.ubuntu.com/ubuntu/dists/natty-backports/restricted/source/Sources 404 Not Found [IP: 200.236.31.4 80]

W: Falhou ao buscar http://br.archive.ubuntu.com/ubuntu/dists/natty-backports/universe/source/Sources 404 Not Found [IP: 200.236.31.4 80]

W: Falhou ao buscar http://br.archive.ubuntu.com/ubuntu/dists/natty-backports/multiverse/source/Sources 404 Not Found [IP: 200.236.31.4 80]

E: Falhou o download de alguns ficheiros de índice. Foram ignorados ou os antigos foram usados em seu lugar.[/HTML]

Thanks in advance.

[oshunluvr]

Yes, remove all references to previous releases.

I think it's at:

/etc/apt/sources.list

[PaulW2U]

Yes, remove all references to previous releases.

I think it's at:

/etc/apt/sources.list

... especially important when they're no longer supported or "end of line".

[geoaraujo]

Well, thank you all.
That warnings are no longer appearing.