How do I record that one company now owns another — move it on the chart?

On Accounts → Company Structure → Chart, switch to edit mode and drag the company's card by its grip onto the company that should own it. You confirm the date, the percentage and the consideration before anything is recorded; the old holding is closed on that date and a new one opened, so the history is kept and nothing is rewritten. Dragging a card among its siblings only reorders them — the PDF prints in the same order. The transfer shows on History with who did it and when. Pick a date in Structure as at to see the chart as it stood before the move.