Do vouchers offered by EU airlines for Coronavirus cancellations turn into a full refund if not used?

If your flight has been cancelled, EU law gives you the right to choose between a refund, the earliest flight available or a flight at a date of your choice (article 8 of Regulation (EC) No 261/2004). See this EU help page.

At the time of writing this answer, the Alltalia English language help page about the situation states that you have that choice if your flight has been cancelled:

Passengers who have had their flight canceled

All passengers in possession of an Alitalia (055) ticket whose flight has been canceled can request the following by 31 August:

• To change your booking for another trip taking place by 31 March or within the period of validity of the ticket

• A voucher, valid for one year, to fly to any destination serviced by Alitalia.

• A refund of the price of the ticket or of the remaining value of the part of the trip which has not yet been completed

That page also says:

Passengers who no longer wish to fly as per the schedule indicated on their ticket

All passengers who purchased a ticket issued by 30 April for a flight departing from 23 February to 30 June can request the following by 31 August:

• To change your booking for another trip taking place by 31 March 2021 or within the period of validity of the ticket

• A refund with a voucher of equal value to the ticket purchased, valid for one year, to fly to any destination offered by Alitalia.

If your flight hasn't been cancelled you can accept the airline's offer, try to negotiate or wait until the flight is cancelled when you are legally entitleed to a refund.

Some customers are demanding refunds before flights are officially cancelled. But airlines aren't legally obliged. Right now, airlines are trying to appear very flexible. But they won't cancel flights and issue refunds unless they think it's in their best interests or they are compelled (in which case they will demand subsidies from the national government or EU).