The current Debian stable , or any future Stable version, IE; stretch or Debian 9
would go on it's own drive, or VM
The Old stable, Wheezy, Debian 7 is what I keep on the "work station" so to speak.
So if you want the best of both , you could go ahead and upgrade your testing, and when Debian 9 is actually released, that would be your final upgrade.
Install and start a new testing, on a VM or another drive, ...
As far as I know, there is no "official" transition period or time,...
by bester69 » Im very bad trying to explain myself in foreign language
You do better on that then me on that,
Immediately after the release is announced, and actually released, there probably is no difference between the packages, but anyone involved in development could possibly submit new changes, or a new package to testing
at any time, after "testing" is unfrozen,... So if someone likes to follow testing,
and they have changed the sources to Stretch, or stable, they would miss the most recent changes,...of course if and when they switch back to testing, a simple update/upgrade would catch them up as well,
I prefer to keep the "SID", and "Testing" on different drives, I use 2 methods,
Virtual Box VM's and also portable USB drives,with the boot able version on it.
In the past there has usually been a announcement posted on the mailing list,
and somebody posts it here, usually about a week before the new release is released, this gives users a week to change their sources.list if need be.