"When defending, convert all crit results into hit results and place all damage that your ship would receive beneath this card. Once there are 4 damage cards beneath this card, discard this upgrade and all damage beneath it. All Excess damage affects the ship as normal."
So it converts all crits to hits when defending. But the standard ruling is that attacker modifies dice before the defender. So I would roll the attack dice and then they would convert the crits to hits, and then I could use target lock to re-roll, convert a hit to crit with toreth, photon torpedoes battle stations to crit, etc.
Can they convert the crits to hits again after I've converted the attacking dice, to be immune to crits, or will they be able to take crits to the Hull.