What will infantry school be like as a prior service SGT from the marines that already has a combat deployment under their belt goin army ?

Your instructors will know exactly who you are, day 1. Shut up and only step up when you're asked, cause you'll be chosen quick overall... don't ever tell these guys, "in the marines..."

You will be expected to LEAD! In basic, 1969, we had a re-tred with a CIB from Korea. He was expected to demonstrate what he had learned 16 years earlier. What I liked about him was the matter of fact way he handled field problems and his steady calmness enabled us to focus on the important stuff rather than the BS.

There is always something to learn.
I have taught classes, and I have taken endless classes. Many on the same subjects. There are always those who come to me with questions on the subjects that I am considered a Subject Matter Expert. With every class I take, I try to learn something. Find something different to put in the tool box, for later. I even learned some new things in my last months before retirement.

Army tactics and procedures are often quite different than those of the Marines. Everyone knows this. Learn the Army way. Once you have done that, you can try to inject items you thing might be performed better. But you should already know, that you have be on the same sheet of music, or you are wrong. Don't be that guy.

Every instructor is different. Every instructor has the potential to give you something new. Some insight on how or why certain things are done certain ways.
Stay open minded
I even learned some new things in a marksmanship class in my last few months before retirement.

Guess what? You are just going to be another dude. What were you expecting? A red carpet.
I was teaching Digital Theory and how to troubleshoot digital circuits for the US Army. My new MOS required that I go through that course. I had to take the tests. The very tests that I wrote and on top of that they made me teach the class. So guess what. You are just another Army grunt to be. Enjoy.

As an 0351, when I transitioned to the Army National Guard, I did not have to attend basic training or an AIT and my Marine MOS transfered over as 11B right off the bat. As I joined the guard as a Cavalry Scout, I did have to attend an MOS reclassification course for 19D (and eventually a 11C reclass as well) . I will say, some kind of Army customs training would have been benificial, but I figured it out eventually OJT.

The army drills might trigger some PTSD on you.

I went from USMC Infantry to Air National Guard and had to do the new MOS school / switching branches thing. I agree with what most people advised, go into it with the mentality that you're learning their culture; "When in Rome, do as the Romans do" and all that.

I thought going through the student experience all over again was fun. It was nice to just sit back and play dumb E-1 for a little bit and have no responsibilities other than maintaining myself. Like someone else said, the instructors will be rotating the junior soldiers in leadership positions to teach them, but the juniors will look up to you for advice and mentorship.

Just have fun with it. Hope this helped. Good luck!

You will be treated like a dud going through infantry school, everyone else will be too. How long, don't worry, 11-Bush is always open at the divisions.
While in the 82nd ABN 2/505th Inf, our XO was prier Marines, and he was one of the best officers we ever had, much better than the newbie Lt's out of West Point.
You'll fit right in the unit.

My question is, your a Marine why go army? Once a Marine always a Marine. SF !

You should have no problem with the physical component, you may need some liquid proof bandages for biting your tung, as you will need to do you don’t step on it. Many tasks will appear similar, but there are enough differences that will will note to make it interesting.

You are just another Soldier, accept that and you may find that the Drill Sgt. May elevate you to a trainjng leadership role where you can hone your skills in leadership more suited for the Army. Again, much the same just enough to keep it interesting. If they do not, or you are not interested, be careful what peer role you take on outside the scope of official leadership. These young former civilians will look up to you. Stay within your boundaries and bite your tung.