Yes, you can do that anytime yourself in the panel. To update your plan, simply go to DH panel --> Billing --> Manage Accounts --> Edit, select the new plan (payment period) and click "update my account". There is no downtime and no additional fee for it. However, if you switch from a short period plan to a long period plan, you need to pay up the balance to make a full plan.
Now all the hosting plans in DH have same features. The only difference is the period of your plan. Whether it is monthly, yearly, 2-year, 5-year, or 10-year. The longer the plan, the more discount you have. Some people go for monthly plan. Some go for yearly plan. But it is all up to you.
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