It turns out it's reather trivial to do, just make sure all entries in one's sources.list point to jessie instead of testing:
--- /etc/apt/sources.list.bak 2015-04-27 22:23:17.664326851 +0200 +++ /etc/apt/sources.list 2015-04-27 22:27:33.559723639 +0200 @@ -1,10 +1,5 @@ -deb http://ftp.cz.debian.org/debian/ testing main non-free -deb-src http://ftp.cz.debian.org/debian/ testing main non-free - -deb http://security.debian.org/ testing/updates main non-free -deb-src http://security.debian.org/ testing/updates main non-free - -# testing-updates, previously known as 'volatile' -deb http://ftp.cz.debian.org/debian/ testing-updates main non-free -deb-src http://ftp.cz.debian.org/debian/ testing-updates main non-free +deb http://ftp.cz.debian.org/debian/ jessie main non-free +deb-src http://ftp.cz.debian.org/debian/ jessie main non-free +deb http://security.debian.org/ jessie/updates main non-free +deb-src http://security.debian.org/ jessie/updates main non-free
and run the usual
apt-get update
apt-get dist-upgrade
