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Most Domain services have a feature where you can forward the new domain to the old one, but keep it within an invisible frame so that regardless of what someone clicks within your site, the URL bar will always read yoursite.net. This isnt the optimal solution, however, just the easiest one.
If you are using the same server space with the new domain as the old, you should be able to open yourxoopssite/mainfile.php and change the URL of your site on line 39. Make a copy of this file before you overwrite it in case anything goes wrong.
Hope this helps :)