There are two separate issues here:
1. You cannot 'deregister' an Office licence registration;
2. In the event that automatic registration fails on the new PC on the grounds that the license is in use, you can revert to telephone activation.
You will not be required to purchase Office anew - unless, perhaps, you tried to re-register an Office version for which support is no longer provided. That doesn't apply to Office 2007, however.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]