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You can't do this anymore. Apple Canada only allows 14 days under "Buyers Remorse" policies to allow this. I experimented (in this case, to see if I would like an iPhone, which I did) with a 6S before I exchanged it for a 6S Plus and this was before the 14 days was up. I paid the difference in price for that.
So no you cannot do the exchange with the price difference in between the items. Far too late for that.
Question: How much does it cost to exchange a 3 month old iPhone 8 to an iPhone X?