You can't. Don't sweat the term "owner" too much. In the case of a custom form, it's more accurate to say that the "owner" is defined as the person who created the form--and you can't change history. If it's particularly important for someone to own a specific form, then you'd have to get that person to make a copy of the form, and reapply it to everything. I recommend against this action by the way--it would be easier to put them in a group that is allowed to access and modify the form, and apply that group to the form sharing tab, and just move on with your lives. -skye