Document Sequence

Dear Gurus,

I have defined a document sequence (with the length of 13 digits), and assigned to Several Categories in GL. While assigning i have also given end date as '31-dec-2011' now the problem is sequence should not be more than 12 digits.

I want to define and assign a new sequence. but the end date field is grayedout.

Can some one tell me, either

how to reset- the initial value

or how to assign a new sequence to the above said categories.

Your valuable response is highly appreciated.

Thanks and Regards

Jithendra