Hello,
since right now KDE SC 4.12.4 packaged from the pre-release tarballs is available in the experimental-snapshots suite of the kdenext repository.
There are no problems expected with this upgrade but if you find something wrong, as allways, you can reply to this thread explaining your issue or question.
The official release is tomorrow, so probably tomorrow this KDE SC is going to be moved to unstable.
Cheers.