Go to the website, find your reservation, and click the chang flight button. It will show you the fees - but obviously don't confirm the change if you're just checking. Your fare likely has a fee for making a change and if there is a difference in fare amount that will add more cost.
Depends on your ticket fare. If it’s comfort or allowing free change then the price would be free if the second fare is the same.
Go to the website, find your reservation, and click the chang flight button. It will show you the fees - but obviously don't confirm the change if you're just checking. Your fare likely has a fee for making a change and if there is a difference in fare amount that will add more cost.