Paul, you are thinking rationally and there is no place for that in this discussion.
As I said above, I believe Disney will try to implement tiered pricing on multi-day tickets. Tickets bought prior to these new rules will be honored. But you already have to buy your tickets well in advance in order to make FP+ reservations.
Tickets bought after the new rules will be subject to those rules. For example, I was in Paris last summer and bought a 1-day ticket in advance for EuroDisneyland. They sell three different 1-day tickets like the new 1-day policy implemented at WDW. Value, Regular, and Premium (not their real names but how they were priced). I bought a Value and used it on a slow weekday. Regular covers slow weekends and slow weekdays. Premium covers high times plus all slow times. If I had bought a Value ticket and tried to use it at a Regular period, I would have had to pay the upcharge to get it to the Regular price.
Segue to WDW with multi-day tickets and tiered pricing. In your example the September tickets would not work until you paid the upcharge to effectively pay the December prices. And given that multi-day are good 14 days after first use, you will have to pay for a Premium ticket if you plan to use ANY (but maybe not all) of those ticket days in Premium period. That's copying the EuroDisneyland tiered pricing methodology described above.
Let's say you will be there beginning December 12 which is a Value time and Premium starts on Christmas Day. If you plan to visit a park on a Premium day (say Christmas Day) even just one day, I believe Disney will force you to buy a Premium multi-day ticket because of the activation window (12th+14 days=26th of December). They're not going to bother with trying to prorate how many days are Value or Premium because plans can change.
So the new "go-to" is that tickets will follow the rules in place when they were purchased. And you can always use a more expensive period ticket during a less expensive time, but not visa-versa.
Folks may not remember but up through 2007 or so, the room/campsite rate for your entire stay was the rate in effect THE DAY YOU CHECKED IN. So what I did once (and others did frequently) was for a Christmas visit (or other peak time), check in on the last day of Regular pricing (say December 19 at $67 a night for example) because Holiday pricing went into effect for arrivals beginning December 20 (at $99 a night for example). But Disney "fixed" their systems to allow them to charge different rates for different dates. And although this example works when rates drop, Disney still implemented day by day pricing because the net effect was likely more money for them.
More money for them. That's what this is all about.
Bama Ed
PS - in my signature the December 2007 visit to see Christmas decorations was the last year you could play the game of "check-in day rate is the rate for all days".