migrating alerts from sharepoint 2003 to sharepoint 2007

Hi all

we are working on a requirement of migrating sharepoint 2003 content to sharepoint 2007.

the approach which we are follwing is : retriving the data from sharepoint 2003 using built in web services and  importing it into sharepoint 2007 using sharepoint API

using this approach we were able to migrate content other that alerts..

Issue: We were not able to retrive alerts of all users in sharepoint 2003 using alerts.asmx.(what ever the crediantials we are using to invoke the webservice that user related alerts only were able to retrive.)

Webservice which i am using is alerts.asmx(GetAlerts webmethod) .

how to retrive the all user alerts ?

any suggetions are appricated.