This one took a while to figure out.
Let's say I have an existing recurring task, let's say it repeats every Saturday, and I want to instead complete it early.
Per the advice below, which does work, I can drag the Saturday recurrence to an earlier day (say, Thursday). Now, I can complete both the sub-tasks within the task and the task itself in the new Thursday copy.
However, and this is the confusing part, the task from which I calved off the Thursday copy (i.e., the main Saturday task) is mirroring the changes I make in the Thursday copy. Further, the Saturday task is no longer editable. This is unexpected.
If you wait a day (now it is Friday), the Saturday task "unlocks" and "resets". By this I mean it is both editable again, and the checkboxes for the task and sub-task no longer mirror the Thursday copy's state.
This behavior is a bit confusing, to say the least. For now, I can manually drag a task to make a copy and then wait a day to have the original behave normally. However, the experience is confusing and could be improved by separating the behavior of the original and the copy at the time the latter is created.