What is the difference between 為替 and 両替? Both mean "exchange". When each of them should be used?
両替 is "exchanging money"... it can be in the same currency, and often is, but can be different as well. You'll see 両替しません on signs on registers around town, telling you they won't make change (from yen notes to coins). Similarly, you might see 業務両替禁止 or something to that effect in arcades on their change machines, telling other businesses not to come in and make change.
為替 on the other hand, entails quite a few meanings, but it's easiest to think of it as changing the underlying denominator. It can mean cash to stamps or other monetary devices, but I think the easiest is thinking of changing one foreign currency to another.
When speaking in fiscal terminology, I always heard 為替リスク or 為替レート when hearing the FX rate or FX risk of some monetary unit. One example was my life insurance policy in Japan that I paid/received in dollars, even though it's domicile was Japan. I even Googled 両替リスク and the first hits that came back were 為替リスク. (Extrapolate that as you will - maybe worth a dig). For what it's worth, every time I went to a bank to change money, the term used was always 両替, but I believe that's short for 外貨両替.
Hope that helps.