I'm going to try to make the next update not break your 1.5 saves, but it will result in situations that won't make any sense. Say you're on day 100 and you get the intro to the Sara story line. The interaction will play as if it's the first week of school.
There are also may be situations where I would simply have to guess at what events have previously taken place or what choices have been made as I haven't got a variable tracking every little thing that happens. Sure, that's something I could do in the future, but I often end up taking the plot in directions I hadn't anticipated. Adding variables tracking literally everything is time consuming and, well, results in just really inelegant and unwieldy code (and mine is a sloppy enough already). I'll try to be better about adding tracking variables that may be useful for future content, though.
Also, in the future I may add significant events that are tied to specific days (say prom, or some sort of Christmas event) so if you've already passed that day in your save there's no good way of making that content available. This, again, causes the problem of having to guess at whatever the outcomes of those events might have been.
Finally, I may have cause to change a class definition. I believe it is possible to add properties and methods to a class at runtime but there are various problems with that approach which may or may not be an issue. So we'll see. I'll try to avoid breaking saves in the future but it may not always be possible.