How do you account for a step acquisition where control is achieved?

When you cross from a non-controlling stake (e.g., an associate or FVOCI/FVTPL holding) into control, IFRS 3 treats it as a business combination achieved in stages. At the acquisition date you remeasure the previously held equity interest to its acquisition-date fair value and recognize any resulting gain or loss in profit or loss (any amounts previously in OCI relating to that interest are reclassified as if the interest were disposed of). The consideration for goodwill is then: fair value of the previously held interest + fair value of new consideration transferred + amount of NCI, less the fair value of identifiable net assets acquired. So goodwill is measured once, on the whole, at the date control is obtained — not incrementally. The classic trap is forgetting to remeasure (and run the gain/loss through P&L) the stake you already owned, or measuring goodwill only on the newly purchased tranche.