On January 9, those with dated tickets will no longer require park reservations. As a result, though, those guests will no longer have the assurance of a spot in the park that park reservations afforded them. This could be significant on the busiest days of the year, such as New Year's Eve.
In light of that change it is probably a good idea to remember how capacity control worked before park reservations. If I recall correctly:
The first level of capacity control was closing the parking lot for the park that is approaching capacity. I suppose they were hoping that, once diverted away from the parking lot, those guests arriving by car might voluntarily choose to visit a different park. (I remember being affected by this once many years ago, when Tricia Yearwood was performing at the Magic Kingdom that night.)
At the next level, ticket sales are stopped. Of course, today they would be able to stop ticket sales for specific parks (and for single-day park hoppers).
At the next level, they limit entry to re-entering or park hopping, or first-time-that-day-entry by guests of a Disney hotel, or one of the dozen Official WDW Hotels (the ones that get Early Entry). This is the first tense level, since guests might be facing the prospect of being turned away from MK (for example) on the last day of their trip, because they didn’t enter before it reached this level of attendance. At the next level after that, they tighten it to just Disney hotel guests.
At the next level after that, they stop all first-time-that-day entries, even Disney hotel guests, therefore limiting entry to re-entry and park hopping. After that, they stop park hopping as well. And of course the final level is no more entries, not even re-entries.
How will it work in 2024? It’ll be interesting to see: Disney hasn't provided details about how park reservations and capacity controls will work for 2024. While it is unlikely, it is possible that we could get into 2024 with some of these questions unresolved, and perhaps even with Disney finding that they left a hole that they need to plug.
One thing we know is that park reservations will remain for guests with undated tickets, which includes all annual passes. (Park reservations will probably also be required for admission similar to this year’s Florida resident Summer Magic ticket, as it is valid on most any three or four days over a four month period - effectively a limited-use partial annual pass. I don’t know where the dividing line will be, but I suspect they’ll choose a certain number of days, probably 21 or 28, and require park reservations for tickets valid over that amount of time or longer)
As such, the first open question is whether the current quasi-guarantee of entry, currently afforded to those with park reservations, will continue, or will passholders be required to have park reservations but could still then be turned away if the park for which they have a park reservation is at capacity. That would be cruel, but without park reservations controlling the entirety of park capacity, it is a possibility.
The alternative would be that Disney uses park reservations to set capacity for passholders, while counting other guests entering the park for the purposes of asserting capacity control associated with the remaining portion of park capacity. That would be seen by both passholders and ticket holders as unfair to them, but for diametrically opposed reasons. Disney isn’t shy about putting provisions like that in place that some guests will find offensive, but they typically don’t put in place such provisions when different sets of guests will abhor them for diametrically opposed reasons. [Either all guests will not like the provisions for the same reasons, or some guests will like the provisions while others don’t.]
What might make sense (but I doubt is going to be the reality) is Disney announcing that park reservations are only applicable (for passholders) prior to 2pm, implying that the quasi-guarantee of late entry with a park reservation is no longer valid, or perhaps just explicitly saying that passholders have to enter before 2pm for their park reservation to assure their entry. (As things are already, after 2pm, passholders don’t need park reservations, except at the Magic Kingdom on weekends. That extra condition, regarding the Magic Kingdom on weekends, makes this whole situation a bit of a mess.)
Regardless, guests (especially annual passholders) should be prepared for how things will work differently in 2024 with regard to park capacity.