Upgrade Reserved Instance

Is your RI type Standard or Convertible?

1. If it's Convertible - you can use the RI exchange feature to upgrade your RI. When exchanging an RI, you can choose the TARGET RI attributes (in your case I understand it's just a larger size), and you'll be prompted to cover the cost difference for the remainder of the term. So, if you purchased your original RI with any upfront payment type, you'll just have to pay the difference for the new RI for the remaining days of the RI term. And for any monthly payments for your RI, the pricing will change automatically for the remainder of the term.

2. However, if it's Standard RI, you will not be able to upgrade it via modification process (exchange process is only available for Convertible RIs). If you have a Linux RI with Regional scope and shared tenancy, then your RI is size-flexible. It means that if you resize your m4.large running instance into m4.xlarge - then your RI will be still covering 50% of your running instance. Remaining 50% of your new instance size will be charged on-demand. UNLESS you purchase an additional m4.large RI to cover the rest of your running instance-hours. Note that this will only work if you have size-flexible RIs - which is unlicensed Linux/UNIX (no RHEL, SUSE, Windows, etc...) Regional scope and shared tenancy.

With Standard RI - when searching for the offerings to purchase an additional m4.large RI, you can find some of the offerings from RI Marketplace (it will be in the same interface as other RIs in EC2 console); these offerings from RI Marketplace work the same way and provide the same discount, the main difference is that they are sold by other customers, and they will have shorter term. You may be able to find the RI, for example, for 11 months instead of 12.

If nothing else works - open a billing support case from your account where you have purchased this RI, and ask Support team if they can provide any assistance with this upgrade.