As far as I know, you play the December championship at the level of you current tier.
For example:
a master-tiered player wins a bunch of credits in the virtual tour (as a master), and is advanced to a tour master in November.
to qualify for the December championship, said player would need to be in the top 70 of all tour masters.
this scenario could work out to be in your favor since you were able to accumulate points against lower-tiered players over the course of the virtual tour...
however, since you tiered up, you get the tougher competition of the higher tier in the championship.
(also keep in mind that credit purses are smaller for lower tiers, although the top tour master would have no problem qualifying/competing at the legend tier)