I am using OpenERP 6.1. In this version I think you can not update report (sxw) by updating the related module (via --init or --update). If you have several database to update, this is what I will do :

• save updated sxw file
• use xmlrpc to object and call execute with model: ir.actions.report.xml and method: upload_method. This is the same method that is used by OpenERP Report Designer.

ref :

• https://doc.odoo.com/6.1/developer/12_api/#xml-rpc-web-services
• https://github.com/odoo/odoo/blob/master/addons/base_report_designer/base_report_designer.py#L45