Retailer Price Setting for Existing Inventory in the Event of an Increase in Price Paid for the Good
At the time period in which you buy the new retail goods, the cost of having bought the old inventory goods is now a sunk cost. Your pricing decision I don't think should be based on the price of the old inventory. The new point where marginal cost equals marginal benefit (assuming competitiveness) is now the price of the new stock (more expensive).

Without competition though, the result will probably change, since the quantity you supply will now matter in affecting demand.