Swap git submodule with own fork

The submodules are stored in .gitmodules:

$ cat .gitmodules
[submodule "ext/google-maps"]
    path = ext/google-maps
    url = git://git.naquadah.org/google-maps.git

If you edit the url with a text editor, you need to run the following:

$ git submodule sync

This updates .git/config which contains a copy of this submodule list (you could also just edit the relevant [submodule] section of .git/config manually)

There might be a way to do it with only git commands, although the submodule system seems a bit incomplete (e.g see the instructions to remove a submodule)