They'd probably not take it lying down. There's a chance this results in firings and the eventual decision to make it a 'white' job. Could blaks in other positions fill the same role? I don't know - probably, to a degree, but the Pullman porters were uniquely well placed.
It'd also be a blow to the Chicago Defender. Without its unofficial distribution network along the rails - pretty much all going through Chicago - the paper probably won't attain the same status in the southern black community. Its campaign to draw black migrants to Chicago would also be hampered.
I'm not sdure how much of an impact all this would have, but it could certainly change initial patterns. If, for example, the New York Amsterdam News or the Boston Guardian replaced the Chicago Defender as the black population's paper of record, the migration might be more pronounced towards the East Coast rather than the industrial Midwest. If its distribution went through ports via black sailors and longshoremen, migration patterns could change. The dialect of the Carolina coast or the Bayou could become stereotypical 'black speech' to New York media figures. I don't know enough about the political differences between the papers to guess at further changes, but I'm sure there'd be some.
What does a black boy in Chattanooga in 1910 dream of growing up to be if the Pullmann porter is white? A captain's steward in the merchant service?