I guess my post was not clear enough. Of course I'm aware that the current stable CSSU won't replace any testing packages. But assuming that there will be a stable CSSU v2 (or v3 or v4...) some day that includes all packages that are now in testing, switching from testing to stable now should give you a system with stable CSSU when v2 is released. By that time testing will have received lots of updates so switching to stable has to be done before these updates are made in testing. If that doesn't work it would imply that there are packages in testing that will never reach stable. Is that the case?
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!