Andrew Smith - 6 years ago
We currently don't have any plans to offer this feature. As a work around, you can create a new object for the detail and migrate the data either as part of an install script or through a post install operation in your app.
The problem with allowing this is that existing objects lose their OwnerId field and also their [ObjectName]Share table in the API. If a customer has built integrations or customized the app to use these fields/objects then the package upgrade will fail. In addition, certain customizations are not possible on detail objects that are allowed on standalone (lookup) objects. This again complicates the upgrade.
We'll continue to re-evaluate this feature as new capabilities are added to the platform that might permit us to allow this.