.@naquadah007 We're actually going to convert SoO to the new raid system in 6.0, so current Flex will just be Normal and won't be going away- Watcher (@WatcherDev) February 3, 2014
While we knew these changes would be coming in 6.0 via screens shown at BlizzCon 2013, what we didn't know is that this format will be applied to Siege of Orgrimmar, as well as all raids going forward into the new expansion. A later tweet by Hazzikostas pointed out that there will be plenty of advance notice before the change goes live, and there will also be more information on the topic at a later date. Considering we're just now seeing patch 5.4.7 on the PTR, we shouldn't expect Mythic modes to be dropped on us immediately.