We have good tools for this. A Googler can create a patch that upgrades a library and run tests for all projects to see what breaks. The project owners review the changes. Of course, some upgrades are easier than others.
In some ways this is similar to what Linux distros do, but sharing common source control, build system, and test runner makes it easier.
But I find it hard to believe that one person can go and fix up all the projects he's hardly ever heard of that depends on that library.