They've changed the terms of your employment. But you don't have a contract so you're screwed.

Can you honestly say you've done your present job well. Have you been getting positive feedback about your performance? Don't BS yourself. If you were the employer how would you view your performance?

Do you know your boss well enough to be able to talk to him about the change? Tell him you're not complaining, you'd just like to know why the decision was made. Is it because you've not been doing what or as well as, they'd expected? How can you improve if they're dissatisfied?

Maybe the company is changing it's focus? If you look like a "comer" they may feel you'll be a bigger asset if you get good at trading and arbitrage. Maybe the sister company is a little weak and they think you might help plug a leak or two.

One suggestion for the next job (and there will be another), confirm anything important said to you during each step of the interview process. Just a note thanking the person for the meeting and touching on the points you discussed and your interpretation of what was said. It's not a contract but it helps keep the air clear and cut back on a lot of possible misunderstandings.