As far as I'm aware Biomes are now saved and shouldn't shift with future changes to world generation.
To get back to the OP's original question, does that now mean that new creations of the same seeds (numerical) before and after 1.8 will still only generate the same biomes they do or is there a chance that the seed, newly generated, will contain the new biomes. I've been noticing that the random seed strings have been getting longer and, sometimes, when I've typed in a number from earlier updates, the game seems to change it to a longer number now (in much the same way it translates words to numbers). The results of this seems to be that some numerical seeds are not possible to generate again since they "flip" to a longer set of numbers during the process.
If this is the case, is this perhaps what is rendering some old worlds unloadable after each update?
The Meaning of Life, the Universe, and Everything.
Title Update 31 for Console Edition has been certified by 4J's snapshots, that armour stands, water monuments, guardians, new fish, Mesa biomes, new enchantments, new fishing treasure, slime block, savannah biomes and endermites will be coming! Also if you can leave part of your world ungenerated, so you have the chance of new biomes!