A nil VAT return needs to be submitted even if there are no transactions to be reported for that period. This requirement applies to annual summary VAT returns and quarterly VAT returns.
Regarding amendments to previously submitted data, corrections can be made to the annual VAT return that has been submitted by filing an additional declaration and ticking the option Correttiva nei termini.
When a corrective return is submitted after the deadline, a difference is made between those corrections filed in the following 90 days after the due date and those filed after 90 days have passed following the annual VAT return deadline.
Further details can be found on section 4.1.1 of the Annual VAT return instructions published by the Italian tax authorities.