There is no universal rule; you must check with each bus line you are considering as to their specific policies. That said, I am not aware of any unreserved buses on the busy Boston-to-New York City route. If you make a reservation for a 7pm bus and wish to travel on an earlier or later bus, you must therefore exchange your ticket.
Depending on the terms of the ticket you purchased, this may entail additional fees, and may need to be done a certain amount of time in advance of the trip. The more established services offer ways to change reservations at the terminal, online or using a mobile app, or over the phone (though different fees may apply); independent operators like some of the smaller "Chinatown bus" companies may have more limited offerings.
- Megabus charges fare difference plus a $3 trade-in fee plus a $2.50 (or $7 by phone) new reservation fee if the exchange is requested more than 24 hours in advance of departure. The trade-in fee rises to $7.50 if done only 3 to 6 hours in advance.
- The BoltBus terms state that changes are only available 24 hours or more in advance, with a $3 rebooking fee plus fare difference.
- Greyhound's website and contract of carriage don't seem to specify an advance notice requirement, but the change fee on a non-flexible ticket is a stiff $20 on top of fare difference. For comparison, the lowest economy fare I see for this route is $9–20, whereas an "economy extra" ticket allowing free same-day changes is $35–39, and a fully flexible ticket is $43–47.