In early April the Union reorganized its cavalry and artillery. They did not create cavalry or artillery brigades; they simply shifted the locations of various troops. This took men away from areas and commanders that they knew and caused some confusion when the battle began. Part of the reason given was that it would make some units, particularly the cavalry, more knowledgeable of the whole area instead of the small portion they were patrolling.
The Union could have reorganized at any time in the weeks before the battle or not reorganized at all. There would have been less confusion on the Union side during day one. Whether this would have had a great impact remains to be seen.
Verdict: This probably does not qualify as a critical decision because it is unclear how much impact this had on the course of the battle. This is one of those decisions that I flip flop a bit on. Overall I don't think this had much effect but its timing happened to be very poor and if it hadn't happened the cavalry might have been a tad more effective, especially in scouting operations on April 4th and 5th. A tough call but for now not a critical decision.