Depends on the situation. If the system automatically rebooks you, an agent once told me it searches for availability based on your original fare class. Then again, if you get an agent to do it, anything can happen. Was supposed to fly JFK-IAD-SAN couple months back with upgrades confirmed with CR-1s. JFK-IAD took a major delay for crew rest and lovely agent at the JFK RCC rebooked me JFK-SFO-SAN keeping me in a premium cabin (C). Plus I got the CR-1 back. YMMV.