Change runtime from Python to Go in App Engine standard environment
Specify a runtime as well as a new value for version.  When deployed you'll have an older version that is Python and a newer version that is Go.  There won't be any downtime (same as when deploying a newer version of Python).
Rather than trusting links/docs (that may be out of date or not 100% exactly what you're trying to do), why not create a new GAE-Std project for testing purposes and try it yourself.  Having a GAE-Std test project is good for testing new function (especially by other testers who won't have access to the dev environ on your laptop).
I have a website on AppEngine that is 99% static. It is running on Python 2.7 runtime. Now the time has come to evolve this webapp, and since I have almost none Python code in it, I'd prefer to write it in Go instead.
Can I change runtime from Python 2.7 to Go, while keeping the project intact? Specifically, I want to keep the same app-ID, the same custom domain attached to it, the same SSL certificate, and so on. 
What do I have to do in order to do that? I surely have to change runtime in the app.yaml. Is there anything else?
Bonus question: will such change happen without a downtime?
I'd be grateful for any links to documentation on exactly that (swapping runtime on a live app). I can't find any.