this is the code that I used to change a fresh 12.10 install into Raring

sudo sed -i 's/quantal/raring/g' /etc/apt/sources.list
followed by
 apt-get update && sudo apt-get dist-upgrade
I do not know if something like that will work in May of this year. The repositories have to be available and have to have a name that can be referenced. Up until now the repositories have the code names of the releases. Continue testing. Some of us think it is fun. I am not sure that I am there yet.