After properly setting up
/etc/apt/preferences as described above you can begin the upgrade.
Note that tracking the testing distribution of Debian can have the
side effect of delaying the installation of packages containing security fixes,
since such packages are uploaded to unstable and only later
migrate to testing.
Debian package management, Chapter 6 for
the basics, and see
troubleshooting, Section 6.3.2 if you encounter problems.