I found a simple way to easily change your flatworld preset in Minecraft Pocket Edition! This requires a computer, an iPhone/iPad/iPod touch and no Jailbreak is needed! Please forgive me if someone else has already made a post like this, I didn't see any and I legit discovered this on my own.
First off you will need to download a simple file editing tool onto your computer. All should work fine, but I suggest using iExplorer.
Plug your iDevice into the computer and open iExplorer
Once open, click "continue with demo" in the bottom right and navigate open the subcategory on the right called "Apps".
Go down your list of apps and find Minecraft, once in Minecraft, go to Documents>games>com.mojang>minecraftpe Once there, drag out options.txt into a desired keeping folder on your computer.
Finding the Preset
Open options.txt in a standard text file opener. Now your going to see a bunch of text, I won't tell you what this all means, but if you think about it you can figure most of it out as the options that you access in game. Note: Near the end are a bunch of Dev settings, these cannot be accessed in game and I am not entirely sure what they do, maybe someone smarter than me can figure that out.
The option you're really interested in is the 4 numbers in that look like this: [7, 3, 3, 2]
Changing the Preset!
If you've played PC edition, you'll probably have already gotten the direction this is going. All the blocks in Minecraft have a number that represents them in the game code, Bedrock is 7, Dirt is 3, and Grass is 2. See where this is going? You can find a full list of all the Data Values here. I suggest making a backup of the original options.txt file just in case you ever want to revert or you delete something by accident. You can make it up to 255 "blocks", the standard block limit of MCPE. I haven't found a way to use blocks that have sub numbers (as seen in the picture reference on page). Glitches I have encountered are that if you make your blocks to low, then you will fall and the game will crash before hitting the ground, and if you make it too high it will cover everything in a layer of grass. If you stay within the height of my presets it should work fine though.
Floating Sand (Oh Yeah)
Saplings (Eventually Crashes, just 'cause)
Once you're done just drag the options.txt back into the file in iExplorer and hit replace! Open up Minecraft and check it out!