I don t understand how you printed out tickets at home. Greyhound does not offer that option. You can ORDER the tickets on line...then they will mail them to you or you can pick them up at the ticket window when you go to the station. According to the site below and to my experience also, you cannot print out the tickets at home. Maybe they offer that option in cetain locations...but not on their national website.
Exchanging them: fine idea