Any time an event happens on a specific day in this game, it will always accommodate for existing saves that are beyond that point. The event system is set up so that the event will keep trying to trigger until it succeeds. In other words, the new event in this update will trigger the first morning after you load your old save.
The system even handles events that clash with each other. If two events try to happen at the same time, one will get priority (according to an order I've decided at design time) while the other will wait until its next opportunity to trigger.
If, at any point, I will require an event to happen on a specific day, we've come to a point in the story where I'll have to invalidate all saves anyway. That's still some way off, I think.