No, the current build map is not going to be in the same spot it is in forever. Once 1.9 rolls around, or we're on more stable ground, a new PvP, Survival, and Build will be established to allow people to take advantage of the new terrain generation and biomes.
No!! my brand new mob tower! my pride and joy! you cant.. just leave the mobs on for legacy survival, or I will /ragequit, say a rude word to my self and bang my fist against the table! you don't want that happening, right?