Yes, you can make changes even after the book is uploaded to Family Memories.

All projects (including books and calendars) will be listed on the Projects page in your account, you can delete the version you do not like from the projects list. Once the project is uploaded, it cannot be changed at the Family Memories site; however, you can make the changes on your own computer and re-upload the file to the site. In other words, any changes can only be made on your own computer; once it is at the Family Memories site, nothing can be changed.