Why should Greece destroy their cash euros? Leading economists in my country (Sweden) recommend and predict that (in the case of default) the Greek government will destroy all the euro bills in the country. Making holes in them and stamp them to become drachmer which are maybe 80% or 90% less valuable than they were. There seems to be about €80 billion (M0) in euro bills in Greece, about a third of one year's GDP.

What would the upside be with destroying this fortune? Wouldn't it be better to for example use those €80 bn to pay the debts? Do any prominent non-Swedish speaking economists give the same advice?

In Sweden former heads of the central bank, the highest government officials, economists at several financial firms and the publicly profiled academic economists (including professors in the committee which chooses so called Nobel prize winners in economics) all talk about destroying the cash euros as if it was some kind of solution. Have they all gone raving mad, or what am i missing? How could it even be done? How force people to destroy their own cash? And I think they have to cut away more than 1/3 of the bill to maculate it, or else they can be exchanged for new euro bills.
Using Google translate I have read some of the articles you referred. It seems to me that the problem the Swedish economists are talking about is that Greece will need its own currency (this is debatable). However it takes a long time to get currency in sufficient amounts.


  Before the 2003 invasion of Iraq, citizens in many parts of the country began to ditch their own currency, believing that it would soon be worthless as all notes carried a depiction of Saddam Hussein, the country’s president. De La Rue, the world’s largest printer of banknotes, began work on new cash. “In the case of Iraq, the issues may not have been dramatically different from any other currency change; but the timescale was,” the company said. “Working out the details … is all very well when you have a year or two to think about it. Make that a month or two, and things start to get interesting.” 


Source 

Hence the recommendation is to use what they have at hand: euro bills. But if their euros would be the same as other euros, then the exchange rate between a Greek euro and a 'normal' euro could not go over 1, because you could always take a Greek euro abroad and use it there. So if the government wants the exchange rate to change (this would increase competitiveness and this is the biggest advantage Greece could get out of a new currency) they will have to modify the existing euros they have. For example they can stamp holes in them. You can read about such speculation in more detail


  During a “euro pause” the Greek Central Bank is granted a three-year licence by the European Central Bank to create euros that are non-exchangeable for other euros.
  The euros in Greek ATM machines have “GT” (Greek Transactions) stamped prominently on them. In Greece, these euros circulate normally but a Greek individual planning a transaction outside Greece will need to convert their GT euros to non-GT euros at a market rate.


here. 

It is debatable whether having a national currency two months ahead of time is worth the cost of 80 billion euros. However it might also be that the ECB will demand 80 billion euros from Greece if it leaves the Eurozone. (I am not sure whose property the cash allocated to Greece is.) In this case, stamping the euros may satisfy the ECB and give Greece their temporary currency.