Yup, this is almost certainly true for tickets purchased directly from Disney. They know exactly what you paid, and they'll charge you the difference.
The murkiness is around tickets purchased from a reseller. Currently those tickets are valued at current gate price, because Disney does not know when you bought them, only when they were sold to the reseller. So the only upper bound they can put on the price you might have paid is today's gate price. The expiry dates might change that, because Disney can reasonably assume that you won't have paid 2019 gate price for a ticket that expired in 2017. They still won't know (or care) what discount you might have gotten from the reseller, but they'll be able to put a reasonable upper bound on what you paid.
If they don't do this, then they'll create a giant loophole allowing indefinite hedging against price increases. Which would be great, but IMO unlikely, since presumably that's what this move to expiration dates is intended to avoid.
That said, I'm not sure I really understand why Disney *has* gone to this new model. Usually businesses would rather have my money today than tomorrow, and expiry dates on tickets discourage this. I guess they're just assuming that if I don't buy tickets now, I'll be happy to pay 30% more in two years? At that rate of increase, I can see why they'd be content to wait.
The only other thing I can think of is that it might be an accounting thing. But since the future use of those tickets doesn't cost Disney anything, I don't see any reason they'd be required to report it as a liability, and thus no reason to want it off the books. But I'm not an accountant, and business accounting rules are complicated. So, maybe.
