It was, indeed just what Sherman wanted:
"About 10 A.m. of that day (July 18th), when the armies were all in motion, one of General Thomas's staff-officers brought me a citizen, one of our spies, who had just come out of Atlanta, and had brought a newspaper of the same day, or of the day before, containing Johnston's order relinquishing the command of the Confederate forces in Atlanta, and Hood's order assuming the command. I immediately inquired of General Schofield, who was his classmate at West Point, about Hood, as to his general character, etc., and learned that be was bold even to rashness, and courageous in the extreme; I inferred that the change of commanders meant "fight". Notice of this important charge was at once sent to all parts of the army, and every division commander was cautioned to be always prepared for battle in any shape. This was just what we wanted, vis, to fight in open ground, on any thing like equal terms, instead of being forced to run up against prepared intrenchments; but, at the same time, the enemy having Atlanta behind him, could choose the time and place of attack, and could at pleasure mass a superior force on our weakest points. Therefore, we had to be constantly ready for sallies."
--W.T. Sherman, "Memoirs" 1875
It is really easy to make decisions and be a General when your enemy WANTS to come at you or you KNOW they have orders to do so.
It was the best news he could have received.