Attempting to change ownership of an item in ArcGIS Online returns the following error message:
Error: Unable to change the owner of items that are shared to a group with the update capability.
The item is shared in a group with the item update capability enabled. This capability allows other members of the group to edit and update an item's metadata, content, symbology, and more.
To circumvent this issue, stop sharing the item with the group temporarily, change the owner of the item, and re-share the item.
Alternatively, you can use a Python script to automate the process of transferring the ownership of items that are shared with "Shared Update" groups. The steps are outlined in the script contained in the following article: How To: Automate ownership reassignment of ArcGIS Online items shared to groups with the Update capability