Answer:
You need to split the traffic to your test version.
Step-by-step explanation:
Google app engine has a built-in feature that allows you to split a small fraction of your traffic to your test version.
You can split traffic using an IP address, so when the app receives a request, it can hash the Ip to a value between 0 to 999, and use it to re-route the request.
This way you don't need to stop your service during your tests and also avoid any risk involved with it.