We do support upgrade to any of the higher packages anytime during that billing cycle.
So for example if you purchased Hosting Plan I in January and would like to upgrade to Plan III or unlimited packages, just email us and we will generate you an invoice for the remaining period in that payment cycle.
Note 1: The invoice for upgrade is generated for the remaining period of that year. So if you have used Plan I for 6 months, later if you request upgrade to Plan II you will have to pay for the remaining 6 months on Plan II, not the whole year.
Note 2: You will not be able to upgrade Plan II, Plan III that host more than one domain to Unlimited Plan I as it can only host one domain. In this case you will need to upgrade to Unlimited Plan II.