The pre-patch for Classic Wrath of the Lich King is scheduled to release sometime on August 30. When this happens, TBC Classic will be phased out and the game client will be changed to the WotLK version.
Before that we have an update that will add Guide Viewer support for this changed version of the game and add some content that will be available later, including:
A Beginner's Guide to Death Knights
Inscription 1-375 Career Guide
Talent Advisor Update
New Stormwind map update support
To receive updates, you need an active Zygor Elite membership. The download will be done with the current WoW TBC Classic content installation on the client, so if you already have that content installed and updates enabled, then you're ready to go. We are planning a small client update that will change the installation title to "TBC/WotLK" to avoid confusion.
The rest of our Wrath of the Lich King: Classic content will be closer to the full launch of World of Warcraft Classic, scheduled for September 26th. You can find cheap WotLK Classic Gold information on our promotions page.