Seems counterintuitive to me. This would only make gains when they bought the new gold before selling the old, or when there's some arbitrage going on between Gold/USD, Gold/EUR and USD/EUR.
If they first sold the old for USD, then bought the new for USD, with a rising gold price, they'd miss the price-gain during the time between the trades, when they held the USD. It'd be a loss, not a gain.
If there's some arbitrage going on, then I highly doubt that brings $15B gain. The differences would have to be huge.
I think the (author (AI)) writing that article is simply mixing up stuff. I think this gain is not a cause-effect of the conversion, merely the gains from rising gold prices on the gold it holds over that period.