There are some limitations regarding buses, I think this type of renaming does not work (see BUS combine over hierarchy and others).
As a workaround I think you would need to split the bus into separate members which can be reassigned correctly.
There is at least one wishlist item on the subject https://bugs.launchpad.net/kicad/+bug/1419146, whether that gets implemented or not depends on a developer taking an interest.