Remote Repos
To add a remote
git remote add origin new.git.url/here
To change the remote repo (see this stackoverflow question):
git remote set-url origin new.git.url/here
you may need to
git push --set-upstream origin master
Diff With Remote Repo
git diff master origin/master
Submodules
See the git book
Force Pull
To pull and overwrite any existing files: (see here)
git fetch
git reset --hard HEAD
git merge origin/$CURRENT_BRANCH