The upgrade to 6.7.3 from 6.6.x is painless and should have been done years ago - LOTS of bugfixes and security fixes have been included since 6.6.1 was released more than 3 years ago
It's available in the members area still, along with the old threads 6.5.5 and the current threads 7.1, you access them from the same page, no extra charge if your members area access is current for 7.1
