It's going to take more than just replacing the leadership. Every e-nothing gets that same training; whether illegal orders are coming down from the president himself or their immediate supervisor or anyone in between, they'll be met with resistance.
Now, there are plenty of trumpanzees among the ranks who will put their loyalty to Agent Orange over their country and constitution, and do whatever he says no questions asked, just as it is with the non-military population. So, if we see an 'order 66' event, what's going to happen is a SHIT TON of in-fighting within the ranks to the point that our military readiness is going to nosedive, leaving us vulnurable to other forms of attacks, but I don't see troops actually hitting the streets like that in any organized fashion.
Again though, what I'm worried about are the cops. They're already completely loyal to that shitstain, and have a long and regular history of extreme unnecessary violence and not actually following the law. And they're already mobilized and infused into every single city in the US. Combine that with the civilian trumpanzees and we've got a very large force that have been itching for a civil war for like a decade at least.