Your PayPal details are used in the various checkout server behaviors that you have on your checkout pages. You should start on your checkout page and edit any server behaviors with PayPal in them. The other pages are confirm, and pp_confirm.
Double click on the PayPal server behaviors and update anywhere with the PayPal account details. Also, you should make sure to enter your new updated API credentials.
If you have trouble with this you should backup your current checkout pages and then generate a new set of them with the checkout wizard.