Here's the breakdownI need to go back and rewatch because I still don't understand how his shit works. Could he change his condition thing to Ants instead of Spiders if he wanted?
The condition that Kurapika placed on Chain Jail (and maybe the index finger chain?), was that he can't use it on anyone besides the Spiders. This is a normal vow to himself that "proves" his determination to himself, and with how Nen works, that's what makes his Nen stronger. If he breaks this condition, it just means his determination wasn't as strong as he claimed, which can make his Nen a lot weaker or even remove it. If he adds an asterisk (that he can also use it against the ants) to this condition, it will cause a far lesser but similar weakening effect.
Then Kurapika used his own Judgement Chain to set another condition on himself, that if he breaks the above condition, he will die. This made his Nen a whole lot stronger, and it's something he can never lift or change.