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 ?

Now I'm not going to sit here and tell you that I know what you're going through and everything because I don't. But what I can tell you is probably 90% of the people in that class with you aren't going to run home the first second they get and try to get their hands on your SRB or something. None of them will know what you dont tell them. If you are scared of getting special treatment for being a Marine stuck in a newly obtained Army uniform then don't provide anyone with the Intel that you are a Marine. It's pretty simple, like don't ask a question if you're scared of the answer. If I were you I'd just sit there and be quiet, just try to soak up how the Army does things because I can tell you that their infantry does things a lot different then we do. I don't think it would be possible for a Marine who has went through ITB to go to infantry units in the Army and not be a liability to be honestly. Probably get in a lot of trouble for doing things that's totally normal in the Corps but Army will look at you like you got a dick growing out of your forehead. Just buckle down and graduate the school. On graduation day if your instructors don't seem to know your name then you are good to go and you weren't the guy talking about the glory days in the Marines the whole time. Just kick everyone's asses at marksmanship:)

SGT, I would keep my mouth shut and my eyes and ears open, and get into the Army DNC drill manual, because if you'll going to drill soldiers, you have to know that Marine Corps COD and Army DNC is way different, because the periptery of command and command of execution is on different feet, then what your used to in the Corps. I was a SGT coming into the Army, and going to BNCOC was a BIG challenge, where you have to forget what you know and learn it again, and the instructors will challenge you also, but put the extra effort in with others that know what you're going through and you'll find that the Army is way easier, and you only have to run 2 miles, and do push-ups in their APFT test, so get in the books, and get familiar with their terminology, because it's a new language, and an adventure.

I did that too. Easy and disappointing

When you are still active in the marines when you change branches, you will retain your grade. Whether you will be an E-2 or a sergeant during training I don't know. I was stationed at Ft Jackson when we got a new training cycle in and we had no Drill Sergeants on Duty. A former Marine Drill Sergeant was in the new training cycle. He organized the trainees, and had them in formation Monday Morning when the drill sergeants returned from their training. He was immediately promoted to E-3, and upon finishing basic training, he was given his previous grade, even though he had been out of the military for 6 years.
Be yourself, do your best, and you will succeed.

You are just another dude. Check that ego, drop the attitude, listen more than you talk. That's why you've got two ears and one mouth. There are some basic facts that you need to know before you go.
1. Nobody gives two shits about your deployment. I guarantee your drill sergeants and instructors have more combat time than you.
2. Nobody cares you were a Marine.
3. It will take you the same amount of time as every other brand new shiny grunt in the making.
4. You ain't Superman, you ain't special. Deal with it.

The worst thing to do is ask for special consideration

Judging by the responses below it sure seems the things you might have loved about the Corps just ain't gonna be there for you in the Army. Draft mentality has never been shed there, and going to visit my brother in law at Fort Bragg in 1992. sure as hell cemented that firmly for me.
He was a Spec 4, and I was a Corporal of Marines. I watched him get an ass chewing near his barracks by his NCO of the day. No big deal but when the Sgt turned his glare on me, he didnt appreciate me telling him I wasn't anywhere in his chain of command, and I was on leave.

Yep, never painted rocks in 3/6 Weapons Co., or any of that other b-s that the Army makes their troops do. He was 82nd Airborne, btw. Did not reenlist. (Shocker, right?)

I think if anything you should be sound and fit. You should have the P T test. And pass it. Now whatever your m o s was most likely that will change
And your rank will have a o be discussed. Other than that.

You will be treated accordingly. They will know you're prior service and likely you already have a bit of rank so they'll expect you to be a role model and help set the standards. It's not your job to Manipulate the students as you already know what perminant party is like so give basic answers as you'll never know what each unit is like I was in 10th and 82nd areas so trust me very different expectations. Just be the solider to look up to and not the arrogant I'm props service guy.

Prior service gets treated a lil different but be ready to get the full green weenie. Either way thank you for raising your right hand and for signing the dotted line. Welcome to the United States Army family and thank you for your service.