Normally when you change fical years, there is a short year between the old and new years. Thus you would have a year with one period from Dec 1 - Dec 31, then a year starting Jan 1.
I think Oracle (JDE) support should have more detailed instructions on how to change your fiscal year.
I only did a month end change. I was on a July - June fiscal year with Saturday month-end dates. One year, starting October, we switched to calendar month ends. Because we had posted no transactions to October and beyond, all we had to do was change the month end dates. Your situation will be more complicated.
Brad
World F6.2 - A7.3
OneWorld B7321 - EnterpriseOne 8.12