I'll get straight to the point.
The [Upper Blackrock Spire] and [Leeeeeeeeeeeeeroy!] will become a part of the legacy achievements. Blackfathom Deep and Razorfen Kraul will remain as low level dungeons so the achievements tied to them will not be of concern in the next expansion. Sources are here and here.
However, I understand Blasted Lands will become a new zone to quest at level 90. So I'm wondering what will happen to the [Blasted Lands Quests] achievement? Thank you all.