Tomas,

Polling the server frequently for changes would significantly reduce the performance of the server and would affect the actual users using the product. For these kind of notification purposes we have work item change events. See http://blogs.msdn.com/narend/archive/2006/07/27/679571.aspx if you are not familiar with how web service subscriptions work. If you subscribe a webservice to all work item changes, then you get the changed fields in your event and you can migrate to target system. This could be almost instance and will not affect TFS performance if you use only the changed fields data sent to you in the event xml. You could still do less-frequent polling like once a day or so to retry failures in migration you had in your webservice and to verify all bugs got migrated.