Updating several devices one at-a-time is quite time consuming - and also I might miss some of them.
Is it possible to batch update all devices?
I could think it would be nice to be able to select all devices in a group (in the device list) and select “Update OS” or to have a setting that just says “Always update to newest OS”.
This is implicitly set for all devices, except that there’s no timeline for when a device will be updated exactly. So it might take a (long) while, but they should all be upgraded by the time the next release is ready.
Not with the dashboard itself (yet?), but there’s an example “app” that does this: If you go to the github page and follow the link you can add a minimal app to your info-beamer account. It probes all devices and updates them one-by-one to the latest release within its release channel.
Ah - ok. Maybe the question was not correct: Would it be possible to update all (selected) devices from the test channel to the stable channel? 
The “app” already does that for one special case: If the device is running testing and there is a stable version available with a later version number, then it switches the device to the stable channel and that version. I guess that’s not enough in your case?