Enable consolidated billing in AWS Organizations so RI discounts are shared across accounts and Move all accounts under a single AWS Organizations management account to link them are correct because they allow unused Reserved Instance discounts in one account to reduce On Demand charges in other linked accounts.

Enable consolidated billing in AWS Organizations so RI discounts are shared across accounts aggregates usage across the payer family so unused Reserved Instance hours in a member account can automatically offset matching instance usage in other accounts and lower overall spend.

Move all accounts under a single AWS Organizations management account to link them is the required operational step to create the payer family and enable consolidated billing and RI discount sharing across those accounts. Accounts must be linked under the same management account for the discounts to flow across members.

Switch the fleet to Amazon EC2 Spot Instances can reduce costs for interruptible workloads but it does not apply existing Reserved Instance discounts that are already purchased in a member account.

Place the EC2 instances into a cluster placement group only affects placement and network performance characteristics and does not change billing or enable Reserved Instance sharing across accounts.

Sell the idle Reserved Instances on the Reserved Instance Marketplace might recover some spend for that specific account but it does not immediately apply discounts to other accounts and it is not the simplest way to lower overall cross-account spend when sharing via Organizations is available.

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