For the closest historical example of the breakup of a currency union, people are starting to talk about the breakup of the Austro-Hungarian empire, and the resulting chaos as each country “stamped” their currency to make it local. They basically had to close the borders to prevent people from moving currency to the country where it was thought the currency was strongest.
There are about 900 billion euros in cash floating around, so the what to do with the cash is an interesting question.
The Euro bills do have country identification codes (which say which national central bank issued it), so perhaps you should look through your wallet to see which currency you’d get.
The ECB strongly denies that the country code could affect the legal tender status of a note within the Euro zone, and cites Article 3 of the Decision of the European Central Bank of 13 December 2010. However, since this is a decision of the ECB, rather something that is clearly spelled out in a treaty, this could in theory be un-decided by the ECB.
So lets see, my wallet has:
4 x V Spain
1 x Z Belgium
2 x X Germany
1 x M Portugal
1 x P Nederland
If you are feeling paranoid, start holding on to any 50 euro bills that start in X. If you have any that start with Y, you may want to spend it as quickly as possible.