This is a little tutorial on how to change things in your map with NBT. First you'll need to download and install it.. As an example I'm going to take my Apvia map that I started in Beta amplified.
Once you have it installed start it up.
You'll first see a directory tree of all your saved games, Pick which map you want to edit.
Click on the little + beside the world you want to modify.
Then click on the little + beside Data. Clicking on the + expands the directory.
Click on the value you want to change (1).
Then click on the pencil icon (2) and an Edit String box will pop up.
You can change the value in the box (3) and click OK.
Click on the save (4) before you exit.
There are a ton of things you can change on your map.
You can allow cheats or not (allowCommands)
Make the game hardcore or not (in case you build something in HC you don't want to lose..
Change your spawn.
Change your game type: survival (0) creative (1)
Change your seed.
If you're unsure how to change something just open a different map with the option you want turned on. Find the difference and apply it to the map you want to change. For example I didn't know how to make my Apvia map amplified so I created an amplified map and saw that I had to type "amplified" in the generatorName field.
If you do make an old map amplified you will have to expect unsightly edges against where the map isn't amplified. The same thing will happen if you change a seed. Personally I don't care. I think it kind of looks cool anyway.