If you Upgrade to a higher membership plan, you will be charged a prorated amount at checkout to get you up to your next billing period. Your membership will continue at the new rate after that.
If you downgrade your membership, the system will credit your billing cycle and you will not be charged until your extra funds have been exhausted. (Example: If you paid $100 then downgrade to a $10/month plan, you would not be charge for 9 months. At the 10th month, you would be charged your first $10/month payment.)