How so?
On the day you can modify it, either the time or the ride & time.
And once you’ve tapped out not one then you have even more choice.
In theory.
You can only modify a pass to a different time if it is still available. On the whole tier 1's aren't available same day, and most of the rest sell out as the day goes on. That's what I meant.
So sure, you can change Remy to Nemo, but that's not a good trade, IMO. I was also partly trying to draw a contrast from my experience using G+. I wasn't always able to modify G+ passes either, but more able to modify them than I've have using LLMP.
In HS as of 2pm the following are the available options: Aliens 8pm,
MMRR out, MF 8:25pm,
RNRC out,
Slinky out, Rise out, TSM 9pm,
ToT out.
While the tiers open up after you tap your 1st, you are still looking at leftovers. Looking above, I'm slightly surprised to see TSM available. It is probably an afternoon drop that was not available half an hour ago. You will generally NOT see same-day passes for Remy or FEA. The tier 1 you prebook in Epcot is the usually one you get, and you have little control over the time you are assigned. You can maybe swap 11am for 4pm - if you do it 3 days before you arrive or get lucky with a late drop. If you are offsite, you might only be offered Soarin'.
If I was in HS right now and realized I would not able be to use a1:30-2:30pm pass for ToT, my HS alternate options are pretty limited. Mind, I'm just using now as an example. Mostly, when I'm at WDW, mostly if I was looking at modifying passes, it was usually more like 9:30am.
There is also the phantom availability I mentioned. What you can see from home is not often what you can actually book when you attempt to book a pass. MDE often said passes were available until I tried to book them, or else the timing of the pass jumped wildly. What looked like a 1-2pm pass instantly changed to some random, much later time, like 5-6pm.
I also meant that if you entirely miss a time window entirely, say Buzz, you can't rebook that specific attraction again, or modify the time window by yourself. You can book a different pass, but the only option to use the Buzz LL is if a blue tent CM can reinstate the pass.
This last part was a real problem when WDW has/had the Virtual Queues. My VQ group was called for Tiana, but the wait for Tiana was 2 hours, and I therefore missed 2 return windows. (they overlapped 10minutes, and the 1st was just about to start when my BG was called.) Technically, I was off Tiana before then 2nd LL expired, but it was SpaceMtn, and I only had a few min left inside my official time window. No way was I running across MK in the mid-day summer heat.