Minecraft – Is it possible to convert the type of a Minecraft world

In this thread on Reddit, I found a command line script that will generate terrain by repeatedly restarting a server with varying spawn points until the area you specify is filled. This seems like just what you're looking for.

There's also a Bukkit plugin named WorldGenerationControl which can do it on a live server.

Deleting the region file should force the game to regenerate the chunks when they are entered.

Region files are located in a subfolder of the world directory called "region", and have names in the form r.x.z.mcr, where x and z are the region's coordinates. The coordinates for the region a chunk belongs to can be found by taking the floor of the chunk coordinates divided by 32.

You can also use World Edit's //regen command to regenerate specific areas of the map.