Hi there! While we always encourage families to use the curriculum as your servant and make substitutions where needed, that particular substitution would be difficult. The World of William Penn covers the years 1660-1718 which is earlier than The World of George Washington which covers 1732-1799. The World of William Penn is 188 pages (my copy) while The World of George Washington is 349 pages.
We always like to recommend that substitutions be made of a similar length and content/subject when possible. That said, you are the Momma and know your kiddo’s reading ability best.
While there are exam questions at the end of each term, they are not necessarily book specific however in the upper levels, we do take into consideration everything the child has read over the course of the term. As an example, in Early Modern, term 2, one of our exam questions for kiddos in grades 1-3 is “What is a colonist” What colonists have you heard of? Tell about one.” The same day kiddos in grades 7-9 are asked, “Tell all you know about the Revolutionary War, including how it affected citizens-both loyalists and patriots.” While all kiddos will likely pull from the family spines that were read, older students are asked more in depth questions that will require them to pull from all they’ve read over the term.