1) Yes, you will want to re-enable/add all of your repositories, then
run "sudo apt-get update"

2) You just need to save your sources list to an external location.
use something like:

"sudo cp /etc/apt/sources.list /media/ /backup/sources.list.bak"

You can then restore with the reverse. i.e.,

"sudo cp /media/ /backup/sources.list.bak /etc/apt/sources.list",

and you can put the command into a bash script, if preferred.

The only issue is if you are installing an new version of ubuntu, you
will want to retain the new repositories, so you would have to pipe
your text in sources.list.bak into your new sources.list via an
append... which is unfortunately a little above my pay grade without
spending some time on the web to figure out the right commands, and
I'm not sure if that is what you are after, either...