Change of Posting Period

Hi,

Can somebody help me? I want to change set up of the posting period of 2008 and 2009. It was set up as calendar year starting from January and it ends December. I want to change it to fiscal year. 2008 should have posting period from April 2008 to March 2009. 2009 should have posting period from April 2009 to March 2010.

2008 and 2009 have transactions posted already.

Thanks!

Tintin