Cards live in the subject they were written in. To place one somewhere else, write it in the destination subject and remove the original.
Whole subjects move freely, which covers most reorganising.
Export the subject as a .reps file, import it into the class you want it in, and delete the original. The cards survive the trip. Their review history does not, so every card restarts its schedule.
Decide where a card belongs when you write it. Filing is a one-time decision in Reps by design: a class is a course, a subject is a unit inside it, and the card lives in the subject. There are only two levels, so there is rarely much doubt about where something goes.
If you find yourself wanting to move a lot of cards at once, the usual cause is a subject that was drawn too broadly. A subject called Biology will collect cards that want to live somewhere more specific. Cell Biology and Genetics as separate subjects tends to hold up better across a term.
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