What should I do if my Company breaks the 4856 they gave me?

A 4856 is nothing locked in. It is a counseling form that holds no legal weight other than to advise and reflect. It's not a contract. There is no such thing as breaking a counseling.

Command had to swap soldiers around to accomplish the mission. A soldier got hurt and is unable to do the mission required so they take you, healthy and able and swap the two of you. Unless you just enjoy desk duty, you probably got the better end of the deal.

What the hell over. Someone got hurt and the unit needs you to backfill them. Move out and make it happen. A 4856 can be amended simply by flipping the form over and scrawling an update in the follow up block and plan of action.
My question is if you failed a record APFT as stated in other responses; why are you still a Corporal and haven't been laterally transferred back to SPC. Corporal is a big deal as the unit forfeits a SGT on the MTOE or TDA for you get it.

If you have a copy of your counseling, then bring it to your local JAG or Legal NCOs at BDE.
It is a paper trail. He can go back and say why didn't you do this, and you show him that on the counseling, he wrote something else. Most NCOs never close out a counseling which irks the crap out of me. I tell my NCOs to close the last one out before you start a new one. Counselings are not just developmental, they are Event Oriented. Like how you stated you failed your APFT, that was an event. You pissed hot, now your counseling becomes an event. No one really reads the back of the counseling in the parenthesis. Example, (Leader Responsibilities), your leader is supposed to state (written or typed) on what his responsibilities are for you to complete your plan of actions during the actual counseling. Then look further down, on Assessment, "(Did the plan of action achieve the results)?" aka a follow up. That is where you close out. Did it or did it not? I'm sure a POA for failing an APFT is doing remedial PT at 1700hrs Mon-Fri for 30 days. (Example). So 30 days later, you passed your APFT. So now he closes out said counseling and writes in Passed APFT, you both date and sign.
So IF you think that your NCO is breaking his portion of the counseling, he should close it out and write under the assessment on why and, re-type a new counseling stating what you will do. But like I said, see your local Legal or JAG but go through the proper channels first. But missions do have to be accomplished and tasks need to be completed. If you're going to make a big deal about getting out of doing work, then you should reread Duty. Sometimes you just got to Soldier up.