I have multiple times been in the database and swapped item types to ones that inherit an exiting one without loosing all the existing data.
So I thought why not bring this option to the backend?
Explanation:
When you create a new ItemType and select to inherit from and existing one the save process of the new item type should add all the item-ids from the existing ItemType table to the new one.
When you then want to change ItemType type to one that inherits from the itemtype currently active you don't need to be warned about the data being deleted and you skip the delete statements in your code.
What do you say? Possible?