What Attributes Can Be Modified on an EC2 Regional RI?

An active Regional Reserved Instance in Amazon EC2 allows you to modify its scope to a specific Availability Zone, merge or split reservations, and change the instance size within the same instance family on Linux or Unix platforms with default tenancy. This article details the modifiable attributes, the normalization factor rules that govern instance size adjustments, the platform restrictions that limit changes, and the attributes that remain permanently locked unless you use a Convertible Reserved Instance.

Modifiable Attributes on a Regional Reserved Instance

When you own an active Regional Reserved Instance, AWS permits several configuration adjustments at no additional charge. These modifications do not change the remaining term or the expiration date of your reservation.

1. Scope Adjustment (Regional to Zonal): You can convert a Regional Reserved Instance into a Zonal Reserved Instance by pinning it to a specific Availability Zone within the same AWS Region. While a Regional Reserved Instance provides automatic instance size flexibility across all Availability Zones in the Region, switching to a Zonal Reserved Instance removes instance size flexibility in exchange for an explicit capacity reservation in the chosen Availability Zone. You can also switch a Zonal Reserved Instance back to a Regional scope at any time.
2. Instance Size Within the Same Instance Family: You can change the instance size of your reservation (for example, changing from m5.xlarge to m5.large or m5.2xlarge) provided the reservation remained in the same instance family (m5), runs on a Linux/Unix platform without SQL Server or SUSE/RHEL licensing constraints, and uses default tenancy. Note that Regional Reserved Instances already apply instance size flexibility automatically at billing time for eligible platforms, so manual size modification is primarily needed when you are simultaneously converting the reservation to a Zonal scope or reorganizing how your reservations are grouped.
3. Merging and Splitting Reservations: You can combine two or more identical or compatible Reserved Instances into a single reservation, or split an existing reservation with multiple instances into smaller separate reservations. Merging requires that the reservations share the same instance family, expiration date, offering class, tenancy, and platform.

Understanding the Normalization Factor for Size Modifications

When modifying the instance size or splitting and merging quantities within an instance family, the total compute footprint of the reservation must remain constant. AWS enforces this using a normalization factor assigned to each instance size.

For any size modification, the total normalization footprint before and after the change must satisfy the following equation:

\[\text{Total Footprint} = \sum_{i=1}^{n} \left( N_i \times F_i \right)\]

Where \(N_i\) is the number of instances in reservation group \(i\) and \(F_i\) is the normalization factor of the corresponding instance size.

For example, if you have one active c5.2xlarge Regional Reserved Instance (\(1 \times 16 = 16\) units), you can modify it into:

• Two c5.xlarge instances (\(2 \times 8 = 16\) units)
• Four c5.large instances (\(4 \times 4 = 16\) units)
• One c5.xlarge instance and two c5.large instances (\((1 \times 8) + (2 \times 4) = 16\) units)

You cannot modify a single c5.large (\(4\) units) into a c5.xlarge (\(8\) units) unless you merge it with another c5.large reservation that shares the exact same expiration timestamp and attributes.

Attributes That Cannot Be Modified

Standard and Regional Reserved Instances have strict boundaries on what can be changed through the modification workflow. You cannot modify the following attributes on a Regional Reserved Instance:

• Instance Family: You cannot change a reservation from one family to another (such as m5 to m6i or c5 to r5) through a modification. Changing the instance family requires a Convertible Reserved Instance and uses the Exchange process rather than a modification.
• Operating System / Platform: You cannot alter the operating system (for example, changing from Windows to Linux/Unix, or Red Hat Enterprise Linux to SUSE Linux).
• Tenancy: You cannot modify a reservation from default (shared) tenancy to dedicated tenancy, or vice versa.
• AWS Region: Reservations are permanently bound to the AWS Region in which they were purchased.
• Term Length and Payment Option: You cannot change a 1-year term to a 3-year term, nor can you switch between No Upfront, Partial Upfront, and All Upfront payment options via modification.
• Instance Size for Non-Eligible Platforms: If your Regional Reserved Instance runs Windows, Windows with SQL Server, RHEL, or SUSE Linux, or if it uses Dedicated tenancy, you cannot modify its instance size. For those configurations, the only permitted modification on a Regional Reserved Instance is changing its scope between Regional and a specific Availability Zone (or splitting/merging instance counts of the exact same size).

Prerequisites and Rules for Modifying a Reserved Instance

Before submitting a modification request in the Amazon EC2 console or via the modify-reserved-instances AWS CLI command, verify that your reservation meets these operational requirements:

• The Reserved Instance must be in the active state (recently purchased reservations in the payment-pending state cannot be modified).
• The reservation cannot have another pending modification currently in the processing state.
• If you are changing the scope from Regional to a specific Availability Zone, the target Availability Zone must support the exact instance type of the reservation and cannot be a Local Zone or Wavelength Zone.
• All reservations being merged in a single modification request must have the exact same expiration date down to the second, as well as matching payment options and offering classes (standard vs. convertible).