There is an option in Master Financial Calendar:  checkbox “User-Defined Periods”  that may help you.  From the Acumatica Help: User-Defined Periods is a check box that you select to enable the modification of the periods of the latest year. You can edit the end dates of periods; the system will automatically adjust the start dates of the periods.
After you modify the periods and save your changes, if the last period ends on a different date than the financial year does, the system opens the Update Financial Year Dialog Box where you can select the method to update the financial year template.

I recommend trying the changes in a test copy of your database to make sure you get the desired results. Acumatica support/development has assisted some clients with fiscal changes that were more complex.  Perhaps “Plan B” is to open an Acumatica ticket.

Laura

i cant seem to get the User-Defined Periods to become available. we are mid-financial year, but need to insert another financial period. any suggestions?