How are plans billed if changed?

If they switch between 2 plans, say 49.99 PAYG annual and then to 299.99 Unlimited Annual, how is that calculated, how much will they be charged and what happens if people switch frequently? 

I use add-on service or unlimited both monthly and annual options. each add on is a post type where they can pick as many as they want increase reduce at their leisure, worried this will charge people multiple times if they switch back and forth / select montly when they meant annually and get ‘double-charged’.