In a lot of countries, it's not allowed to change an invoice in any way as soon as the invoice has been sent to the customer. You can't even correct a typo.
It has been brought up multiple times before and for that reason I'm shocked that a "stop splitting invoices"-feature hasn't been implemented.
If a client has an invoice that covers renewal of two hosting products but cancels one of the products, WHMCS will simply split the invoice line to a new invoice and cancel that (or the software removes the line from the invoice - not quite sure, but neither of the solutions are valid from a legal point of view).
When splitting invoices, it would be nice if WHMCS did this:
1. Cancel the original invoice
2. Create a new invoice for the item(s) you selected to split
3. Create a new invoice for the remaining items
That way, you don't change any information on the invoice - you just create a new one.