[joerichey]

(Google employee here)

If the change is small or can be automated (i.e. changing # of parameters or function names), we run a script to make the change over the entire monorepo. This enormous CL is then approved by one of the Global owners.

If the change is complex (or non-obvious), you generally introduce the new API in one CL, change each use manually (say one CL per team), and then remove the old API in a final CL. In that case, you need each team to sign off. This isn't too hard in practice, teams are generally expected to approve cleanups.