In case you want to upgrade the pricing plan of the HostX you can do that within one month of the purchase. After 1 month you will not be eligible for upgrading the HostX plans. You need to purchase the complete plan again.
See how does the upgrade works:
If you have purchased the startup plan and want to upgrade to Enterprise plan, you need to raise a ticket or contact us at our customer support chat. Regarding the payment you need to pay the gap amount (Enterprise Plan Price - Startup Plan) to upgrade.
Upgrade will take upto 24 hours to reflect into your account after completing the payment.