Since you're going to a new Server version, you should be re-testing anyway. Ultimately, the choice is yours:
(1) retest/revise with Office 2013 on the new Server, then spend some money upgrading to Office 2013; or
(2) spend some money migrating Office 2007 (for which you probably can't buy a volume licence from MS anyway) and -


(2a) retest;
(2b) not retest and take your chances.

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