Please help with posting period

Hi all gurus, could you please advise how to change the Posting Period settings in B1.

The problem is: We set up the posting period for 2008 by mistake.

Financial year has been set up as Yearly, 2008. So the system shows the period from 01/07/2008 to 30/06/2008.

But now we need to change to Monthly, 2008, and 12 periods.

So we can have 12 months periods for financial year 2008.

When I added new periods, the system shows:

"The entry already exists in the following tables 'Periods Category' (OACP) (ODBC-2035) [Message 131-183]"

Appreciate anyone can help on this.