Given the precedent of Greece (where Euro was introduced after the initial rollout):
- the European Council will determine an exchange rate ahead of time(should be months)1
- Banks will accept the exchange of legacy currencies
- Banks will begin to dispense euros from ATMs, as well as only euros will be available as withdrawals from the bank counters.
- Merchants will accept the legacy currency, but give change only in
- Depending on the country, as was the case in Germany, the Deutsche Mark would no longer be a legal tender on roll-out day but would have to be exchanged at the banks, therefore not allowing both currencies to circulate in parallel.
So, all in all, you won't really lose money in any way when the switch is made and I would advise against speculating on the aforementioned exchange rate unless you have experience and expertise doing so*.
[*] If not made clear enough, following the announcement of the exchange rate, what in essence is a peg, should be strong enough to not allow deviations from it given the switch on a specified close-by future date as that would create opportunities for arbitrage and incentive for traders to maintain said rate.