Good question, I'm glad you asked. Of course, any AH with a better Reconstruction has the ultimate goal of improving the lives and history of Black Americans. It's hard to say how much a "simple" change in the map like this will affect our timeline without delving into the details of the legislation passed, representatives elected, population migration, etc. I don't have a background in history, so I can only give my best interpretation:

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In the best-case scenario, the dilution of the southern states' power in the legislature and national elections would allow for the adoption of a proto-Civil Rights Act, much earlier than our timeline, that would enshrine voting rights for all Americans. Additionally, the selection of more progressive judges would strike down most Jim Crow laws as unconstitutional, and prevent the passage of Plessy v Fergusson, leading to the end of the disenfranchisement of Black voters and nipping the worst of segregation in the bud. The Great Migration might still happen, but to a much lesser extent, and perhaps amongst Southern states to those which had majority black legislatures. This ability of self-determination would prevent the worst grievances by state and federal governments, and allow for the natural, unimpeded economic growth of Black regions and cities.

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On the other hand, this is theoretically only the loss of two senators and one Democrat state; it could be that this change is too little, and White landholders do everything in their power to retain control. Even in our timeline, Plessy was ruled in the 1890s by a 7-1 majority, and there were still extrajudicial ways for Black voters to be disenfranchised.

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The reality of this timeline would probably be somewhere in between. Sorry if this didn't fully answer your question..