well, you can set up a mirrored domain.
The way this would work is, for instance, you'd set up domain.com as a mirror or domain.net. Now when anyone visits domain.com they will see the content from domain.net even though their browser url will show domain.com.
Is that what you want?
Or, possibly you are saying that you no longer want to keep domain.net and only use domain.com? If that is the case it's a simple matter of using a ftp client to move the files from domain.net to the domain.com that you allready set up. If you're running some '3rd part scripts' you may have to update their configuration to work with the new url. Also if you have any absolute (http://www.domain.net/file.html) links to your own files you'll have to update thoes manually as well.
does one of these solutions answer your question?