How did the "silver devaluation" scheme work?
Ok, let's look at the situation more carefully. You have your Trenni silver coins, which have some % of silver in them. The market value of such a currency is bound to the amount of the noble metal in each coin. It's very simple and intuitive: silver is rare and valuable, so the more silver there is in a coin, the more is the value of that coin.

Now, we have a situation where the amount of silver in the coins is going to be decreased. What is going to happen? Well, the new Trenni coins (with less silver) will become less expensive than the old Trenni coins. That means, that it is profitable to save all the old coins you have.

By spreading the rumors that the amount of silver is going to increase, the Medio company is accomplishing two things at once. First, it makes people think that it's profitable to sell their coins now (because they think that after the new coins arrive, the value of the old ones will decrease). Second, it creates a possibility to make people selling their coins for even a lower price than they are now, simply because they want to get rid of the coins ASAP, before the price drops.

Also, if they already have some new coins, they can even trade them for a larger amount (say, 2 old ones for 1 new, people think the new ones are more valuable, remember) of old ones.

This lets the Medio company stock plenty of old silver coins, which as we know, in reality, will be worth more when the new ones are released. That's how they want to make the profit.

The Milone company, after getting the information from Lawrence, starts doing the same thing: buying Trenni coins to get a large stock of them before the new coins are released. After the new coins will be in play, people will, of course, know that they are really less valuable, and the value of Milone company's stock of old coins will increase further.

I hope the explanation wasn't too messy ;)