Based on my experience, I advise you to change your money once you arrive at your destination since you will always get a better exchange rate.
The anxiety of thinking that we can find ourselves in a foreign country without local currency makes us rush and change money through our bank or in one of the airport exchange places.
To reassure you let me say that in the airports of all the countries that I have visited, there are always currency exchange houses available. Sometimes even a small bank branch. Once there, check the exchange rates of the different places. In some countries the change will be the same in all of them but in others it can vary notoriously.
In any case, once at your destination you will always get a better exchange rate than in your country.