Warning: if you really need/want to do this, do it at your own risk, as this guide comes with no support (and be sure to have a backup just in case)

Use case:

You want to modify type hierarchy (ie: type A -> subtype B -> subtype C to type A -> subtype C) or want to change attribute name/type while keeping metadata AND there is a limited number of affected objects (I don’t think you want to try this with millions of objects)

1. Create a new project in Composer
2. Import every affected object as sysobject
3. Delete unwanted type (subtype b) from type artifacts
4. Modify subtype C supertype to type A
5. Add necessary attributes to your subtype (those required that were inherited from subtype B) or modify the name/type of whatever attribute you want to modify
6. Open sysobject artifacts with text editor, modify <typeExtension> node with the required <attributes> elements (remove unused, add new ones, edit modified definitions). You probably want to develop some program that automatically does everything for you. JDK has anything you need without additional libraries.
7. Delete every object instance of types b and c from repository.
8. Drop types b and c
9. Install DAR.