This thread, on FLYER talk says it's OK to change as long as there is space

http://www.flyertalk.com/forum/budge...-miss-bus.html

Though on their website now they say

All tickets are valid for travel only on the date and schedule for which the ticket was purchased. If you wish to change your travel plans, you may have a ticket re-issued for another schedule (subject to availability), and you will be charged a $20 change fee. Please arrive at the terminal at least an hour before departure. Boarding generally begins 15 to 30 minutes before departure. When picking up passengers en route (such as at a rest stop), continuing passengers who de-boarded at the rest stop are given priority to re-board.