1)No, Shadow Priests won't be replaced by Ret Pallys. The reason for giving them both mana regen for the raid is that in case you don't have a Shadow Priest, you can still do the raid with a Ret Pally. Also, Frost Mages are will also give mana back to the raid. Again, Frost Mages aren't gonna replace Shadow Priests, even though their buffs overlap.
2)Why would Warlocks disappear? The only thing I can think of is Malediction overlapping with Death Knight's Ebon Plague. Again, they are both gonna bring that same buff to the raid, but there's plenty of other reasons to take a Warlock(Soulstone, Healthstones, Banish, etc) and plenty of reasons to take a Death Knight(Silences, Off Tank, etc).
What they're doing, to answer both at once, is doing away with the REQUIREMENT of having a Shadow Priest.
Now: You're not on today? Well <censored>. Thanks a lot, now we have to completely re-do our raid setup to maximize healers because they don't get any mana during Illidari Council.
WOTLK: You're not on today? That's alright, we can bring the Survival Hunter and Ret Paladin.
Next day...
Now: You're on? Good, so and so mage sit out because shadow priest mana regen is better than your dps.
WOTLK: You are on? Good, you're a good player and understand the fight. The Misery debuff is a great help too.