If you have confirmed space on the flight pull up the reservation and if it has been cancelled you will see a message indicating that there has been a change and/or the automatic rebooking.

What may be more likely than the flight being sold out, particularly if the flight has been downgauged is that Revenue Management may have decreased the seats authorized for sale to a number equal to or less than the number of seats currently booked, possibly (among other reasons) to allow for a potential further down gauge to an A319 if necessary or to manage weight and balance. If that is the case it will no longer show any seats available for sale but won't impact those already on the flight (in virtually all cases)

(To elaborate: For every cabin on every flight there are two numbers -- the number of seats physically on the plane and the number of seats authorized for sale. In the front cabin the two are virtually always equal -- 16 seats physically in domestic first almost always means 16 seats authorized for sale, unless for example revenue management wants to protect against a downguage to an aircraft with only 12 F seats, then the actual would be 16 and the authorized would be 12.

On the other hand cabins in back are frequently authorized for more than the actual number of seats because the higher seat counts mean a higher likelihood of no-shows and also frequently over sales can be resolved by upgrading into the next cabin... But again if there's a likelihood of a downgauge or other disruption Revenue Management will cut the authorized seat count to minimize the number of passengers impacted.