[IMP] mrp: usability of BoM (lines)

- Task: https://www.odoo.com/web#id=10010&view_type=form&model=project.task&action=333&active_id=131
- Pad: https://pad.odoo.com/p/r.FJILGyANBSVkazkw
`@oldname` only works for pure renames, not when the semantics or the type changes, like here.