Yes, you can upgrade at any time. It is free to upgrade and the only charge would be the price difference in the plan you upgrade to. For example, if you were paying $9.95 per month with your previous plan at Host4Cheap, and the new plan is $14.95 per month, you'd only pay the $5.00 per month difference.
To upgrade : You can simply Login to your Client Area and upgrade to package you want and follow these steps
1. Click My Services
2. Select the Details of Package/Service you want to upgrade
3. Click the Upgrage Downgrade Package and select the package you want.
Direct Link Looks like this : http://www.host4cheap.org/orders/clientarea.php?action=productdetails
Note: The Billing system calculates the amount due using advanced Mathematical Formula to calculate the amount on Prorata basis i.e if you have used 5 months of service and have 7 months still left, it will calculate the amount due after taking into consideration, the amount already paid and time already used.