How would you go about changing your MOS to Infantry?

When it's time to reenlist, yes. Or, you can just pass Ranger School and it's a simple request to switch.
What non combat civilian sector skills do you think you will qualify for with no experience after you switch?

Best bet is at re-enlistment time.

It is possible. The key will be having enough time left on your contract. You will either need to extend your current contract or wait until you re-enlist to make it happen. Submitting a package without extending will be a nonstarter. You will need to have a conversation with your career planner or whatever your equivalent to a career planner is. They will be the ones to outline your path and help work the ropes.

You will likely have to wait until your window for re-enlistment

I did. I starter life in the Army as a 63T. I then reclassed to 11B and went to Airborne School. I then got assigned to the 82nd Airborne Division. I got off active duty. I joined the Reserves and I’ve since reclassed to a 42A and then an 88N.

Reclass in your reenlistment.

Simple fail your AIT and then be a sent to 11B

There are ways to do it but why? Mid point on a standard 4 year contract is 2 years. With BCT, and AIT is 38-40 weeks. Then if you have a security clearance above Secret you will be sitting around twiddling your thumbs until that is cleared and done by CID and the FBI and that could take some time. You could easily spend that two years and not actually have any job experience.

Best bet is to change over your MOST to 11B when you are due to re-enlist. That way it gives you a few years to actually do your job.

Yes, after your first term (3yrs,5yrs, etc), you can decide if you want to reclass or finish your time in the Army and get out. Reclass is one of the options.