We use Decta for all payment processing.
We have a 21-day all-money-back-no-questions-asked return policy, which applies from the date of each transaction.
Simply purchase the plan you want to change to. Your plan will be upgraded and you will only pay the price difference. For example, if you purchase a plan worth $1 and upgrade to a plan worth $2, you will have only paid a total of $2, not $3.
Check out the Premium Games section on FAQ page for more info, including how the 5 games per month for the Casual Pulse plan works.
It's just shorthand for "per month" to save some space. Monthly charges are made every 30 days.