nanowrimo_lj | Is this "wrong"?

I have a dilemma. See, I really want to write a fanfic for NaNo. I've spent the past three years when participating writing my own original story, but I'd like to tackle a fic this year. Problem is, I can't think of an idea. Other problem is, I already have a multi-chapter fic in the works. I was thinking: Could I take that pre-existing fic, and in the start of November, start counting the words from 0? Meaning I would NOT include any of the pre-existing words in the word count, which I definitely know is "cheating". Would that be okay, though? What do you all think?

edit: THANK YOU for the overwhelming response and even advice! You all are so very kind!