Okay if you have a pre 1.8.2 map and it's fully explored, there are not Strongholds, nether forts, mineshafts, NPC villages and the 1.8.2+ Biomes.
If you world is after 1.8.2 but before 1.0 and it's fully explored there will be no mushroom biomes or nether forts. (also you will have a stronghold just not the end portal located in the stronghold)
If your world is new and created after 1.0 you world will have everything in the game at the time
However if you fully explore it when future updates come out you will not get their generated structions and biomes since your world is already loaded.
Now if you have ANY of these worlds with unexplored parts the new things will spawn since the unexplored areas are unloaded.
Ex.
You have a 1.8.2 world with some areas unexplored, now that it's in 1.0 I might find a mushroom biome
or in future tense
You have a 1.0 world with unexplored areas and say in a month the update comes and you explore, you have a chance of finding a Jungle Biome
NVM there are mine shafts but I can't find any.. why?
If your world is created before 1.8.2 and it's fully explored none could generate and yes you need a new world for theme.
But if the world is either 1.) has unexplored areas or 2.) is created after 1.8.2 they might just be hiding! Sometimes they are hard to find
How do i check which one i have like the 1.8.2 world?
When did you create the world? If it was after the patch on Dec 19 then you have 1.0.1. If it was before the 19th but after the last patch in November you have 1.8.2. It's hard to go farther back than that.
Well you can't really unless you remember the time your world was made.
If it was made Pre-November 15th you have a Pre-beta 1.8.2 world
If it was made After Nov. 15th but before December 19th you have a beta 1.8.2 world
If it was made after December 19th you have 1.0 world
However if it is terrain, (npc villages, strongholds, nether forts, biomes) it won't unless the area is unexplored(unloaded) when you update.
Also...
Currently in the 1.0 update there are End Portals so you won't have to make a new world in the next update for the End.
If you world is after 1.8.2 but before 1.0 and it's fully explored there will be no mushroom biomes or nether forts. (also you will have a stronghold just not the end portal located in the stronghold)
If your world is new and created after 1.0 you world will have everything in the game at the time
However if you fully explore it when future updates come out you will not get their generated structions and biomes since your world is already loaded.
Now if you have ANY of these worlds with unexplored parts the new things will spawn since the unexplored areas are unloaded.
Ex.
You have a 1.8.2 world with some areas unexplored, now that it's in 1.0 I might find a mushroom biome
or in future tense
You have a 1.0 world with unexplored areas and say in a month the update comes and you explore, you have a chance of finding a Jungle Biome
Sorry trying to explain best I can
If your world is created before 1.8.2 and it's fully explored none could generate and yes you need a new world for theme.
But if the world is either 1.) has unexplored areas or 2.) is created after 1.8.2 they might just be hiding! Sometimes they are hard to find
When did you create the world? If it was after the patch on Dec 19 then you have 1.0.1. If it was before the 19th but after the last patch in November you have 1.8.2. It's hard to go farther back than that.
If it was made Pre-November 15th you have a Pre-beta 1.8.2 world
If it was made After Nov. 15th but before December 19th you have a beta 1.8.2 world
If it was made after December 19th you have 1.0 world