Target does allow you to exchange diapers as long as you have the receipt and they are not opened.
If they are opened Target will not exchange them but with a sealed box or packages of diapers and a receipt Target will allow you to exchange them for different diapers in their store.
According to Targets exchange policy, you can return or exchange unopened packages of diapers within 90 days of purchase with the original receipt.
As long as Target sells that brand of diapers they can be exchanged for other diapers they sell or a refund.
I've exchanged diapers at Target and other stores that were unopened and even got a refund for diapers that were not opened when I no longer needed them.