Man-rating an existing rocket
Ariane 5 which has flown many times and has a fantastic reliability record, is not man-rated.
If it's ever going to launch humans to space, there are several things that need change.
One of them is the acceleration rate:
Unmanned rockets have a tendency to eccelerate to unconfortable rates just before engine cutoff;
Manned rockets must have a steady acceleration (3 G's) and make the transition between first stage engine cutoff and second stage ignition as smooth as possible.
That's a different rocket!!