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From LL page click on the LL pass, it will look like this next:
From the options you see, click modify. Next page will look like this:
This is the part that isn't intuitive and that Disney needs to update, but click "change to this time" and your page will next look like this:
From here, click modify time and you can see how different time options open up similar to initial booking window before purchasing:
So in practice it ends up similar to pre-booking way to modify, but getting there isn't very clear. Also, after purchasing only one LL can be modified at a time, different than prepurchasing when all three LLs can be adjusted from the same page. I hope this is changed in the future because sometimes we move one time to then adjust a second, and it'd be great to do all at once, like before purchasing.