I'm not really working on Google3 (the shared code base) but I hope the following answers might still be helpful.
Same repo: projects in Google are encouraged to use other project's code. You can only do that if the versions of their dependencies match.
The general guideline is that all projects use the latest version and move in sync.
Breaking Changes: I have never been involved in such a huge breaking change, but I would expect that the conversion is done gradually. Ideally, some teams are also helping in the migration (or sign up as guinea pigs to test the latest versions).
Automatic refactoring: These refactorings are done with a tool called Rosie. Search for "Google refactoring, Rosie" to find some papers/documents about it. One of the first hits was this document: http://cacm.acm.org/magazines/2016/7/204032-why-google-store...
Same repo: projects in Google are encouraged to use other project's code. You can only do that if the versions of their dependencies match. The general guideline is that all projects use the latest version and move in sync.
Breaking Changes: I have never been involved in such a huge breaking change, but I would expect that the conversion is done gradually. Ideally, some teams are also helping in the migration (or sign up as guinea pigs to test the latest versions).
Automatic refactoring: These refactorings are done with a tool called Rosie. Search for "Google refactoring, Rosie" to find some papers/documents about it. One of the first hits was this document: http://cacm.acm.org/magazines/2016/7/204032-why-google-store...