To upgrade and downgrate an account in WHM :
>>Login to your WHM Control Panel and click the "Upgrade/Downgrade an Account" setting on the lefthand side of the screen under the heading, "Account Functions."
>>This page will show 2 boxes :
The box one the left has a list of all the currently hosted domains while the box on the right shows a list of all the user accounts for the various domains. Choose the domain/user account that you wish to modify and click on the button labeled as Modify.
Next, on this page you will see a box with a list of all the packages. Choose the new package you wish to assign to the domain/user account and then click on the button labeled as Change. This will take you to a page listing all the contents of the new package that has been assigned to the domain/user account.