You can upgrade your current contract to a higher or lower plan but, the value of the new contract must exceed that of the current contract. Also, you cannot change the contract duration in between your current contract.
For example, consider you subscribed to a Startup plan for a 1-month duration. Now, you want to modify your current contract, you can choose Growing plan or any other depending on your requirement but you cannot change the duration from 1 month to 12 months in between your current contract.
However, you can change your contract duration on contract renewal.