Assuming you mean a different general is appointed instead of McClellan, the alternate general would have several advantages over McClellan. An alternate general probably wouldn't undermine general-in-chief Scott to get his position - not only was Scott a better general, the bad example this provided encouraged subordinate generals to undermine leaders of the Army of the Potomac. An alternate general wouldn't have shown the gross disrespect that McClellan showed towards Scott, Lincoln, and Stanton; undermining his own position. An alternate general would have had a more realistic view of enemy numbers, McClellan's fears affected much more than just his actions. Those fears communicated themselves to many in the Army of the Potomac's officer corps, who were afraid Lee was "suddenly going to turn a double somersault, and land in our rear and on both of our flanks at the same time", even after McClellan was gone. Those fears also communicated themselves to the Lincoln administration, convincing them that large numbers of soldiers were needed to guard the capitol, a fear that helped deprive McClellan of the number of troops he wanted in the Peninsula Campaign. An alternate general would probably move faster the McClellan, possibly much faster, which would also make for better relations between the alternative general and the Lincoln administration.