I still don't fully understand the motivation for Community Wiki, and why it some questions should be originally marked as such.
If a question is turned CW later on, is the accumulated rep gain for the posting user removed?
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No, the rep gained is not lost if it switches over to Community Wiki. You will not gain or lose any further reputation, however. Answers do not automatically flip over to CW status if the question flips (only new answers will be automatically CW). Those answers that remain non-CW will continue to gain and lose rep even if the question itself is now CW.
The only way to gain rep when a question is Community Wiki is for the answer to have been posted before the question turned to CW (if it wasn't that way originally) or by receiving the +15 from an Accepted Answer.