You currently can't cast Inferno while having an active summoned creature, just like you can't use a mind control cap while having a demon out. You also can't summon a new pet without dismissing the demon.
The Inferno spell should work similarly to the other warlock summons, where it simply dismisses the old pet when cast. You should also be able to summon a new demon with an enslaved one, breaking the enslave and making the enslaved creature agressive to those around it.
You currently can't cast Inferno while having an active summoned creature, just like you can't use a mind control cap while having a demon out. You also can't summon a new pet without dismissing the demon.
The Inferno spell should work similarly to the other warlock summons, where it simply dismisses the old pet when cast. You should also be able to summon a new demon with an enslaved one, breaking the enslave and making the enslaved creature agressive to those around it.