How accounting periods work
Think of the accounting periods as brackets enclosing entries. Reports and evaluations are prepared for this period. If the duration of an accounting period changes, a new one can be created and the old one deleted at any time. Overlong financial years are also possible, as are shorter ones.
When you close an accounting period, all documents and bookings are fixed and can no longer be changed. Except for the entries for profit posting etc., not much else happens - very simple.