But unfortunately the same applies, you cannot even change the Copyright text, as per the license agreement:
All copies of the Software must contain the same proprietary notices that appear on and in the Software, including all copyright notices embedded in the Software, which must remain unaltered from the original and visible at all times, unless by specific prior arrangement with XenForo Limited.
All I can suggest is if you wanted to make such a change is to write a support ticket asking permission to do so. However the License terms are quite clear, so don't be surprised if the answer is still "No."
If I were the owner of the copyright I'd be reviewing the requirement for this copyright link in view of the latest Google algo. The rug from under sitewide footer links has been well and truly pulled, they would be much better to ask for a single page on the site expounding the virtues of Xenforo. I personally would also find this less link spamy and much more professional.