The usual "they're just arbitrarily changing it to be more PC...so shallow and pandering!" barely veiled and/or unchecked bigotry response doesn't even work as well here as it does all the other times that it doesn't work.
This is usually true, but especially in a period movie, including this period and genre, a black woman would provide a vastly different perspective on the same themes and general tone. It's different to be a black woman now, but it was REALLY different back then. Considering a "more of the same with the same characters immediately following the first one" kind of sequel wouldn't even be possible without recasting, this is probably the best route they could have gone for a true sequel. A fresh take that allows the period and genre to be as vital and potent as they were the first time, from a new angle.