It should be possible to "drift" from testing to stable just like one would do in Debian. To do that one would have to change the repository from testing to stable which should be done as soon as possible (given that the current testing works fine for you right now).

The effect would be that at the moment nothing will happen because the testing packages are newer than the stable packages. But as time goes by new packages will enter stable and eventually they'll have higher version numbers than the current testing and replace the testing packages bit by bit until nothing of the current testing is left.
Until the transition is complete (which will only happen if eventually all testing packages will make it to stable) the system will be in a state which is similar to one that is based on stable but has some (at the moment - all) packages pinned from testing.
If you chose that way you should know exactly what you're doing. As a rule of thumb: If you don't know what I'm talking about in ths post, don't do it!