Thinking the hitherto unthinkable

The Independent spells out how Greece might revert to the drachma:

A country could leave the eurozone over a weekend, converting all bank balances into, let's say, "new euros". All contracts written under national law would be converted. So people would still be paid in these euros, prices in the shops would remain the same, property deals would go ahead in the new currency. Some imports and exports would be under local law and others under international law and so there would be a scramble to sort those out.It would be messy, but no more messy than it was in Ireland in 1979. People in Britain (including the many Irish ones) who held assets in Irish pounds and suddenly saw them devalued in sterling terms were pretty aggrieved, but there was nothing they could do about that. By contrast Irish people who had British bank accounts had a windfall gain, at least until sterling fell back again, as it duly did.

There would however be one practical difficulty that did not apply then. Ireland already had its own notes and coins and British ones were quickly withdrawn from circulation. Since there are no "new euros" and since "real euros" would be hoarded, any country leaving the eurozone would have to move fast to get new currency into people's hands. The time-honoured method is to overprint existing notes stating these notes were new ones. I suppose that might happen. In practice it might be easier to wait for new currency to be printed. Memo to travellers to southern Europe this summer: take plenty of spare cash, including pounds and dollars, just in case...

There would inevitably be a devaluation of the new currency vis-à-vis the old, for the whole economic purpose is to enable the country leaving the eurozone to become more competitive.

Not sure it would be quite so easy.  For example, there remains the question of all that borrowing from the rest of Europe by the Greek government and the Greek banks.  Repayment in drachma would amount to default and enormous losses for the European institutions and banks;  but a requirement for repayment in euro terms takes us back to square one.  Then there would be the need to introduce capital controls, preventing the Greeks from sending the euros under their beds to safer destinations in Switzerland, Germany and even the UK.