Minecraft – Remove all chunks except spawn and change world type (from default to amplified)

There are three straightforward ways that you can get the newly added worldgen from any mod to be present in your existing world.

• Go far enough away from your currently explored areas to cause the game to generate new chunks (this doesn't require using MCedit / WorldEdit)
• Use a version of MCedit that knows about (and won't replace with air) mod created blocks to delete already explored chunks which, when recreated, will contain the new worldgen. This has the downside of deleting anything you've build in the chunks, but if all the construction on the map is confined to a small area with the rest of the explored area effectively being 'wild' then it can effective.
• Use mystcraft to generate a new age. With the new age creation mechanics in the latest versions of mystcraft this is more complicated than in earlier versions but still fairly straightforward. The easiest way to create a new stable (vitally important) age for the purpose of getting access to new worldgen is to go into creative mode and get a 'creative spawned' mystcraft notebook that will automatically contain all symbols and then follow a guide such as this question to assemble the right symbols in the right order to get a stable age.

I ended up doing Marco Geertsma's idea. Here's what I did

1. Stored everything I had
2. Logged off
3. When into my players folder, and backed up {YourUsername}.dat
4. Regenerated my world
5. Logged in without restoring my character so I could get the coords for world spawn, as the .dat file seemed to contain my coords too.
6. Replaced the .dat file with my backup
7. Logged in and teleported to the world spawn coords.
8. Rebuilt my house, and off I go!

I also used a different world seed, but you could easily just keep it the same.

Something I cheated at to make it easier for me, was I got a couple of Strongboxes to store all my items, and I could only carry about a third of my items at once.