Activity Web Services

Hi,

Are there any special things to know when creating a web service on Activities?  There are so many rules within the application such as no dulpicate checking so updating activity records via import isn't an option, using list mass update only works if the Activity is not in completed status and the person updating owns the activity record.

1. I need to change the Activity Owner when users leave or no longer need access to activities for thousands of records, and would like the rules of the road prior to writting a web service.
2. Will an Activity Attachment  placed on the activity by the original owner stay with the activity and be accessible by the new owner or is a separate web service needed to change the owner of the Attachment?