Hello,
it seems that there is no /tc regen command so I wonder how I am able to regenerate terrain in a world that has been created with Terrain Control?
Any suggestions?

You can use WorldEdit's //regen, or you can delete chunks using MCEdit. (Use the 'Delete chunk' command in MCEdit, not the 'fill with air' command.)

//regen does not work because all the ores are deleted within the chunk I regenerated.
MCEdit ist not an option because it's a multiplayer live server and we can not offline edit the map.