The regular Light and Fit yogurt - I'm assuming your talking about the non-fat yogurt. The difference is that Greek yogurt usually has less sugar and more protein. Because they remove its whey, it results in a thicker consistency than unstrained yogurt which means thicker, more sour, and higher amount of protein. Non-fat yogurt usually contains more sugar which is less desirable. You can substitute if you'd like. My suggestion would be to choose organic versions either way.
Thanks for the suggestion