Existing workflows don't break because there's the vendor command and dependency management is instantaneous. You can also roll it out gradually as it doesn't have any problems depending on non-module repositories (using commit hashes and dates)
There's a great blog post series too if you're interested in the internals of go modules: https://research.swtch.com/vgo (it's also one of the good counterexamples to people claiming go ignored x years of PL research and history)