Just from a programming and software perspective. All the old code that runs banks and the back end of air travel. It barely runs as it is, do a switch up of years, even leaving the months alone and it would probably freak out. Standadize the months while you are at it and the whole thing falls apart.
Are you old enough to remember Y2K? That required a lot of techs to spend a huge amount of time fixing code that was never intended to see years change from 19xx to now just even consider 20xx.
That is before we go about changing paper Birth Documents, marriage documents, house deads, … Should I go on?