In economics, there is something called the "no money on the table" principle. There's an economic parable about an old economist and a young economist. They go for a walk and see a hundred dollar bill on the ground. The young economist goes to pick it up and the old guy goes "No, don't bother, it's not real". The young one asks how he knows and the old economist says "If it were real, someone would have picked it up."
Moral of the story: If there were a way of easily getting rich quick, with no risks, someone else would have done it, removing the opportunity (kind of a weird idea to get your head around, since the idea suggest that SOMEBODY gets rich quick, it's just someone with impeccable timing.) Currency speculation is something that a lot of people try as a method of getting rich, but it's quite risky and any exchange service will charge quite a bit, so you will need quite a bit of money to be able to make this worth it. Also, although VCE economics teaches you that exchanges rates will go up or down in certain situations, there are so many factors affecting exchange rates, including currency speculation (what you've described), that you have no way of knowing for sure when exchange rates will go up or down.