Still under RAMIS RIS can submit SVAT form4 any time (after the due date).
In practically if RIP updated SVAT Form 6, then again RIP can't write new credit vouchers to RIS, even if the RIS updated SVAT form 4. Because RIP has to cancel his/her form 6 and re-update it, otherwise his return will not match. This cancellation has to do via IRD officers. I think the most convenient method is changing the invoice date and update the possible time period (After discussing with RIP).
If RIS manages to update SVAT Form 4 (by using above mentioned method) after the due date, then it again case issue to RIP. Because, If the RIP file VAT returns monthly basis, then RIP VAT return and SVAT purchase will not match and he/she have to again amend his or her VAT return, otherwise RIP will receive an assessment.