There are two differences between these sentences: verb tense and verb voice.
1) WAS CHANGED is in the simple past tense, passive voice. The simple past tense tells us that it was a single completed action in the past. Even though this happened before the "dropped," we don't need to use past perfect, because "after" already tells us the time relationship between these two events.
The passive voice tells us that the action was done to the subject; someone else did the changing, and the test format was the thing changed.
2) HAD CHANGED is in the past perfect tense, active voice. The past perfect is used to show past events that happened before some other past events, to show time relationships. Even though it's not strictly necessary because of the "after," the past perfect here is highlighting that the format change came before the score drop.
The active voice here is suggesting that the test changed itself; it's the one performing the action. We use this in English all the time: "The test changed." Is it really possible for the test to change itself, logically speaking? Not really. But the GMAT usually doesn't get too picky about these kinds of distinctions, unless the meaning is impossible (e.g. "The test completed." vs. "The test was completed." A test might be said to change, but it certainly can't complete itself).
So, the differences in meaning between these sentences are very subtle. Don't worry - the GMAT is very, very unlikely to test you on differences this subtle!
Ceilidh Erickson
EdM in Mind, Brain, and Education
Harvard Graduate School of Education
