Hi,

it is the same principle that views and fields : overwrite report.

For example i edited a report purchase in my custom module :

1° - create folder my_module/report

2°- copy files .rml and .swx in this folder

3°- create file report/order.py

 class order(report_sxw.rml_parse):
    def __init__(self, cr, uid, name, context):
        super(order, self).__init__(cr, uid, name, context=context)
        self.localcontext.update({'time': time})   
report_sxw.report_sxw('report.purchase.order2','purchase.order','addons/my_module/report/order.rml',parser=order)

4° -create file report/purchase_report.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>            
        <report auto="False" id="report_purchase_order" model="purchase.order"
                name="purchase.order" rml="my_module/report/order.rml"
                usage="default" string="Purchase Order"/>
    </data>
</openerp>

you can use openerp interface and not code :

Settings>>Technical>>Reports and duplicate report then edit this report to openOffice.

Thanks.