If it's not too facile and answer, I guess it's like having whole, semi-skimmed or skimmed milk. They're all acceptable, just that one might be preferable at a particular time, I guess.
Or you could go along the line that each day a slightly differently composed milk is produced (different antibodies etc), so is it ok to give yesterday's milk today?
Yes, breastmilk is perfect for each day it is produced, but still acceptable after that, if that makes any sense. It's not like today you're giving oranges and in a few months time it's strawberries. More like satsumas or something more similar to an orange (sorry, but orange knowledge isn't wonderful
)