I don't know for sure. But I would guess that it would depend on the way the government handled it. Statute law can change terms in contracts, for example. As far as I know, at the time of the euro changeover, legislation was passed that included the requirement that euros replace punts, even in existing contracts. If you and I agreed a contract in 1997 to pay me in punts in 2007, you can't now pull out punts in fulfilment of the contract. The law has substituted in euros, so you must pay in euros, even though we hadn't even conceived the concept at the time we made our agreement.
I'd classify the risk of loss due to such an event as one that is difficult to helpfully guard/insure against. In a list of things to take into account when investing that includes charges, diversification and attitude to risk, I personally would rank worrying about the particular point you make quite low. It could well have a big impact if it happens, but I don't think there is a lot you can do about it in advance.