Nadal will still be the current No. 1 even if Djokovic wins Paris Masters.
The Emirates ATP Rankings period is the immediate past 52 weeks, except for:
(i) The Barclays ATP World Tour Finals, which is dropped on the Monday following the last regular-season ATP event of the following year;
Meaning come Monday Djokovic will have 1500 points less from winning last year’s WTF.
If Djokovic wins Paris Masters, Nadal needs to win at least 2 matches at the WTF to end the year as No. 1.