Greyhound is not fussy like airlines (and Megabus) about travel dates. As long as you have a ticket, you'll be able to get on a bus that will get you there. Bring your eticket and the bus driver (who will check your ticket) will let you on the bus. You don't need to do an exchange. I've traveled on Greyhound for 10 years, and never ever once have I been denied boarding because I'm trying to get on a different schedule than what's printed on my ticket.