@oliviadgrace Proving Grounds serve a different purpose and will be updated for WoD. We think of CMs as a PvE analogue to arena seasons.- Watcher (@WatcherDev) February 13, 2014
Proving Grounds, conversely, do not work the same. While there are titles available, there is no ranking, and they do not offer similar rewards. While Challenge Modes are designed to be challenging content that is for players at their best, Proving Grounds are for learning. This may well make them easier to update for the new systems, as well as the fact that they focus on one spec at a time.
What I'm wondering is whether this means Proving Grounds will have a larger role in Warlords of Draenor. WoW Insider posted in the past about their potential use as an entry requirement for LFG content, and while it seems that Flex raiding will overtake LFR in participation, there is still the potential for such a system to be implemented for the Dungeon Finder.
Maybe the system will also expand to be a better teaching tool than it currently provides for the new level 90s that will inevitably appear on the scene. It has great potential for max-level characters to learn, but that potential currently remains untapped. Tutorial sessions, different areas, suggested spell use, shorter scenarios, all of these could be used to great effect for new max-level characters.