change of calendar

Dear Gurus,

I have a requirement of change of GL calendar.We have defined the calendar

Starts from 01-Sep-2010 and ends on 31-Aug-2011 fiscal year

Now our management is decided that we deploy calendar year to Jan- Dec.

Here now we need to end this calendar as on 31-Dec-2011,Where I need to add the months Sep-11, Oct-11, Nov-11 and Dec-11 for existing calendar.

 and we define new calendar from  01Jan-2012 to 31-Dec-2012 fiscal year   

Can we do this? One more thing what are the impacts we have on the ledger and if you have any workarounds please let me know thanks in advance.