According to View from the Wing:
American Express confirmed that the only change is taht “the $600 is to be used $300 semiannually (Jan – Jun/Jul – Dec)” and that “if you already used the $200 credit for this year, you will now also have access to the $300 credit for the remainder of the year” to use on “a new booking.” It doesn’t matter if you used your $200 benefit in the first or second half of the year. So hopefully you already did that, and now also get this $300!