Moving Activities Between Policies
No way Jose.
There is (surprisingly) no direct link between a policy and an activity. There is a sirface association because the activity screen holds a policy type field and a company/insurer field but does not have a policy line link. If you test this by changing the policy type code in the field on an existing activity, you will see by running an Activity Report that the change is retained and the "link" to the original policy is broken.
If this is important to an agency, the best way to handle it is perhaps to have the original activity deleted and a new one created. But note that this will lose the link between the original and any associated Attachment.