You can always substitute light corn syrup for dark corn syrup, but you won't be getting the full effect. Dark corn syrup has been boosted with the addition of a little caramel coloring and flavoring, and some refiners' syrup. Refiners' syrup is a type of molasses, derived from sugar cane, and is primarily responsible for giving dark corn syrup both its color and flavor.

In a dish as assertive as a peanut butter pie, you might appreciate the extra flavor of the dark syrup. If you have molasses on hand, substitute it on a 1 to 4 basis (that is, 1/4 cup of molasses and 3/4 cup light corn syrup for 1 cup of dark corn syrup.)

In your assertive recipe it won't matter, but the one ingredient light corn syrup has that dark corn syrup does not is vanilla, so you are getting a bit of vanilla flavor whenever you substitute light for dark. We don't think that would be a problem for people generally, but mention it in the interest of full disclosure.