General Ledger Calendar Period - Change

Hi,

I have a case;

Company ABC is acquired by company XYZ,

Company ABC is using Calendar Year from 01st JAN to 31 DEC,

Company XYZ is using Fiscal Calendar from 01st SEP to 31st AUG.

Company ABC is also using Oracle Project Costing and Fixed Assets.

How can i change Company ABC calendar from Calendar year to Fiscal year.

Company ABC, under ledger setup "Number of Future Enterable Periods = 26", so when we try to update calendar, it shows error message that we can only 

We don't want to go for secondary ledger option.

What are the feasible options.

Thanks,