Somebody from Germany lends me late tonight 5 euros, while both Germany and Greece have as their official currency the euro. Tomorrow early morning, euro is no longer the official currency of Greece. I still owe the German guy 5 euros. And I still have in my hands a bank note of 5 euros. I can give it back to him, no problem. Euro is still legal tender for him, while it has become foreign currency reserve for me.
The fact that I owe the amount, does not change its international legal status: it is a bank note that the legal issuer, the European Central Bank, will accept. So it is a perfectly valid foreign currency reserve, if it is held by somebody whose official currency is not the euro. The fact that euro used to be the official currency of the holder, makes no difference in that respect.
So declaring that the legal and authentic bank note that writes on it "5 euros" should from now on be understood as meaning "5 drachmas", and undergo the corresponding change in purchasing power and/or value-stored, (by the help of a hole, say), while realistically assuming that the exchange rate will be abysmal, is indeed a bona fide destruction of value.
Now the question (and the special aspect of the situation) is "who holds these foreign currency reserves?" In normal situations, the private sector has a rather small amount of foreign currency reserves of a country under its ownership. In the situation that we are discussing, presumably a visible portion of these foreign currency reserves are legally and/or physically in the hands of the private sector (legally, if they are in private bank accounts, physically, if they are in their wallets or mattresses).
Why should the Greek state do anything to try to gain ownership over these foreign currency reserves? Well, for one thing, because the newly introduced drachma will be weak, and the state will think that the state's needs are above individuals' needs, as states always do. So if someone is to suffer, individuals have priority in the suffering. It is better to have foreign currency reserve in order to secure say imports of food, pharmaceuticals, or energy, than leave these foreign currency reserves at the hands of individuals to be used for private consumption needs (or even individual business needs). I am describing here, not assessing, evaluating or judging.
So we should consider as almost certain that the Greek state will think of various ways through which it will attempt to acquire these foreign currency reserves.
As regards bank deposits, the Greek Central bank could (forcibly, by law) buy the reserves in exchange for newly created drachmas. (I didn't say "printed" -we will return to that).
As regards bank notes held outside the banks, the Greek state could
a) Accept euros when the citizens have to pay taxes etc.,
b) declare illegal any physical transfer of euros outside the country (we will return to that and to smuggling),
c) declare illegal any transaction between individuals using euros.
So the only legal way to use your hand-held euros would be to give it to the state or to the banks (which will hand them over to the Greek CB).
Now, it is easy to bring up the issue of smuggling, illegal activities, defying the law etc. But everything is a matter of degree. If the state is serious about enforcing these laws, not much of the amount of euros outside the banks will eventually leave the country. It is also important to take into account the fact that these foreign currency reserves are scattered over the whole population. Coordinating smuggling operations given such dispersion, will be a real issue.
Finally, let's turn to the other very real issue that @FooBar discussed, the amount of time it takes to actually print the new money in sufficient quantities. Again, this is a matter of degree. How much actual paper money is needed, in the modern electronic banking/credit-card/commercial-credit environment? I should also mention that the Greek state has an active money-printing facility, since it has been printing euros all these time (as many eurozone states do).
So it is a matter of sitting down and measuring the time-table to print the new money in the quantities actually needed, assess the damage to economic activity because these new money will become available only gradually (while again, taking into account the ability to transact electronically/paperless-ly), and compare this damage to the damage done by outright destructing euro bank notes/foreign currency reserves, by punching holes in them, and count them as drachmas.
Once more, everything boils down to a cost-benefit analysis. And no - I will not venture any "impression" as to which cost is bigger.