With the new mechanic to Call Pet in 4.x, the stated intent has been that, to summon a new pet, we must dismiss our current pet first. If our pet is dead, they want us to resurrect it, then dismiss it, then call the alternative pet.
However, in the middle of a bossfight, that's 10-15 seconds of downtime just to deal with a dead pet. Aside from just leaving it dead until after the encounter, you can work around that design intent by beginning the cast of Revive Pet, waiting for the pet's corpse to despawn, then interrupting the cast. Finally, call another pet that is alive.
After the fight you can then take the time to rez the pet. Dismiss your current one, attempt to summon the dead one, then cast revive.