How hard would it be to switch to the Air Force after serving a contract with the army?

It’s fairly easy. I did it years ago. I started out my career as a Army MP. I switched over to AF Security Forces which is pretty much the same as a MP. The Air Force has much better living conditions, the bases tend to be better, and there are more basing options than the Army.

However, I am pretty sure that you need to finish your Army contract first. At least the active duty portion of it. You will need to talk to a AF recruiter to get the best answers though.

I was an 11BxP and had no problem moving over to the SPs in the Air Force.

Air force bases are bipolar. You're either in beautiful Okinawa, or the frozen wastelands and desert.
Everyone wants to be someone else. Infantry wants to be MPs and medical. MPs want to aviation. Medical wants to be something where they actually get promoted. The grass isn't always greener and every switch has its trade offs. You become an MP and you trade those four day holiday weekends for late night duty shifts and slower promotions. Trading branches you'll trade cultures for a more corporate culture (plus or minus depending on your view) and slower, more difficult promotions.