Yes, you can. The upgrade or downgrade of a VPS means the increase or decrease of RAM size, storage size, number of vCPUs, number of IPv4/IPv6 addresses, network speed or monthly bandwidth.
About the options for a upgrade or a downgrade, you may check the plans listed on the page https://www.ehvps.com/cart.php.
You may submit a ticket and tell us which options your VPS hosting will be upgraded or downgraded to. We will calculate prorated price difference for the current billing period. For a downgrade, we will give you a refund for price difference and then perform the downgrade. For a upgrade, once we receive your payment for price difference, we will perform the upgrade. The new fee of a billing cycle will take effect from the next billing period.