You are eligible to make a new Genie+ LL reservation whenever one of the following four things occur (special rules apply if the ride goes "down"):
1. You tap into your most recent G+ LL reservation
2. You cancel your most-recent G+ LL reservation
3. Your most-recent G+ reservation expires
4. Two park hours (i.e. hours the park is open) pass since making your most-recent G+ LL reservation
MDE will show the "time" when you can make your next G+ LL reservation. That will be either the expiration time or the two-hour rule time, whichever is earlier. If it originally showed the expiration time and you modify the reservation, it will show a new time, an earlier or later expiration time, depending on whether you moved the reservation earlier or a little later, or the time that is two hours after the reservation was originally made. If MDE originally showed the two-hour rule time and you modify the G+ reservation so the expiration time is earlier than that two-hour time, it will now show the earlier expiration time. If it showed the two-hour rule time and you modify to another time subject to the two-hour rule, there will be no change.
Examples: DHS opens at 9:00. Pre-park opening, you make a G+ reservation for MFSR for 9:35-10:35. MDE will show you can make a new G+ reservation at 10:35. If you modify the reservation to 9:15-10:15, the time shown will change to 10:15 (the new expiration time). If you modify it to 10:10-11:10, the time will change to 11:00, which is two hours after park opening rather than to the expiration time of 11:10.
The notion that the time shown will not change if you modify your reservation is only true if both the old and new reservations are subject to the two-hour rule. That has been true since the ability to modify came into effect. Modifying your reservation from one time that was subject to the two-hour rule to another time that is subject to the two-hour rule does not change the time when you can make your next reservation. Example: At 7:00, you make a G+ reservation for MFSR for 11:30-12:30. You can make your next reservation at 11:00. At 10:05, you modify the return time to 2:00-3:00. You can still make your next reservation at 11:00. You do not have to wait until 12:05 (two hours after making the modification).
The above is how the rule has been since modifying G+ LL reservations began. It explains why the wdwmagic poster was confused when the time changed when he modified immediate return times to later times. If there have any recent changes, I am not aware of them.
Edit: The two-hour rule time does not change when you modify a G+ reservation. It is based on when you made the original reservation, not when you modified it. If MDE now shows a different time when you can next book, it is because you have switched from the expiration time to the two-hour time, or vice versa, or the expiration time has been changed.
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