You will be charged for the difference when you upgrade and then you will be charged normally on your next billing cycle date. For example, let's say that you upgrade half-way through your billing cycle. If your plan is $100 per month ($100 is for illustrative purposes only.) and you upgrade to a plan that costs $200 per month you will be charged $150.
Here's how it breaks down:
You will receive a $50 credit for the half of your billing cycle on the $100 plan that you're now not going to use because you just upgraded to the $200 plan and you will be charged $100 for half of your billing cycle on the new $200 plan.
When the next billing cycle comes around in 2 weeks you will be charged $200.