One day, no. Unless there was some sort of loophole in the condition by which changing the one day would allow me to relive my life from that point on retaining all my knowledge.
Obviously the choices that you remade that day will influence the rest of your, and possibly others, life.
nah. i can't really see there being just one that would make all that much difference. i did what i could with the options that were available to me at the time. i'm just not convinced there is anything i could have done better at any one point.