Activating Funds Management Module in the mid of a month

Hi

We are planned to activate the Funds management module in the mid of August 2014 and we are following accounting period (April - March). So the PR and Purchase order created prior (i.e from April to Aug) to the activation of FM module does not have FM account assignment (Commitment item and Fund Center). So kindly let me know what is the standard procedure to update FM account assignment in these PR and PO

Regards

Vivek MG