After some trouble, Blizzard have now enabled free transfers from Season of Mastery for EU servers as well, with the US transfers having started a few days ago. Remember that if you do not transfer your characters by the time Season of Mastery realms close (the date hasn’t been announced yet) you will permanently lose them.
Transfers (Source)
Season of Mastery players can now transfer their characters for free to a Classic Era realm or to a Wrath Classic realm of the same ruleset (Normal or PvP).