Can I change my plan mid-month?

Of course!

If you're currently on The Basic or Moderate you can upgrade your plan to the People's Choice plan anytime during the billing cycle. If you're on the People's Choice plan, you can upgrade your plan to the All-In and the change will take place at the start of the next billing cycle.

If you’re on The All-In unlimited plan and would like to downgrade to a shared data plan, you can request the change anytime, but it will not take effect until the start of the next billing cycle.