Login to the Client Billing Area
Click on the 'Services' icon
Select the Service you want to switch
Click the 'Switch Games' option
Click the 'Choose Product' button on the game you want to switch to.
Click the 'Continue' button
Please note, Currently you will only be able to switch to the base package for the game you have selected. You would have to upgrade the server after switching if you wish to increase the Ram/Slots/Etc
If the new service costs more than your current service, You will pay the price difference between the two based on the remaining time of your service.
If your new service costs less than your current service, you will be allocated account credit based on the price difference and the remaining time of your service.