About allocate_rerating_adjustment business parameter

As per the BRM documentation, when AllocateReratingAdjustments business parameter is kept enabled then BRM automatically allocate adjustments to the items on the "original" bill as part of its rerating process. Having said this, when a "billed" MCF event is re-rated, will the adjustments get auto-allocated to the old (original) bill? If yes, isn't useless to do it as the Invoice corresponding to this old (original) bill might have been dispatched to the end customer already and might be paid as well ? Isn't it right to allocate the adjustment to current open (or on-going) bill object "always" irrespective of the re-rated MCF events are billed or