If you are using something like git.debian.org to share repositories and you have done major surgery that makes that 'master' became invalid this is easy to fix in your local repository:

git checkout new-master
git branch -D master
git branch -m new-master master

but what do you do with the bare repository on git.debian.org? Well you can do:

git branch -m master old-master
git branch -m new-master master
git symbolic-ref HEAD refs/heads/master
git branch -D old-master

of course anybody who was following this git repository now has problems ...