If you already have a hosting plan and existing domain names you can switch the hosting plan that the domain names use. To do this you would need to change your plan through the customer control panel by completing the following:
1. Login to your customer control panel at https://manage.myhosting.com
2. Select All Domains From the Drop Down Menu
3. Click on the Domain you wish to change
4. Under Hosting Services select View Details by Web Hosting
5. Click on Change Hosting