Can you change your mind about how a multi-target burn spell assigns damage after casting it?
Nope.

The Gatherer rulings for Fight with Fire have this to say (emphasis mine):


  You choose how many targets Fight with Fire has and how the damage is divided as you put the spell onto the stack. Each target must receive at least 1 damage if Fight with Fire is kicked.


Fight with Fire is already on the stack when you respond with Moment of Triumph, so it is too late to make any changes to the way damage is divided.