This is important for organizational accounts for a number of reasons. One example would be switching from http to https. A programmatic way would be even better than a point-and-click workflow.
The ArcGIS Online Assistant allows me to see the sourceURL value, but I cannot edit it. Only the URL assigned to the item by ArcGIS Online at time of registration is editable.
Thanks,
Tobias
I should clarify: I only have problems doing this with secured ArcGIS Server services where "credentials are stored with the service item". If you don't store credentials with the service item you can just edit the item and change the url in the Properties section.