18 March 2019
The Code deals with the switching and reconcilation process (among other things), but does not regulate customer invoicing (apart from in several very specialised situations that don't apply here).
There is no obligation under the Code to re-issue customer invoices if you receive revised metering information (for example to replace an estimate or correct an historical metering error), as long are you are correcting it for market settlement and switching.
However you will want to check the arrangements you have with the customer as they may cover what happens if a post-invoice correction is required. Most contracts will look to correct the invoice, or to carry the correction through to the next invoice.
Additionally you will need to consider general consumer law such as the Fair Trading Act , particularly if you end up billing the customer for more electricity than they actually use.