The translation of this sentence in context is "If they start fighting, I'll change my strategy". My question is, why would you choose to use だ in this situation instead of する? It's something that seems to be natural and comes back a lot and I have a hard time grasping. Wouldn't する be more precise? What is the difference between the two?
Edit: After a couple of comments, I will add this because I think it clarifies well the confusion. I'm trying to put myself in a Japanese setting where a lot is omitted and the information you have is "change strategy". Knowing this, and knowing that you're about to potentially take this action, you can either say "It will be" with だ or "I will do" with する. To a non Japanese (my frame of reference is English and French), "I will do" seems like the better option because it adds the information that it's an action instead of a statement. However, it seems like it's the opposite that's natural in Japanese. How would you explain that?