Can I change OS setting on an active saving plan

Yes, you can change the operating system (OS) on instances covered by an active Savings Plan without affecting the plan's benefits or requiring any modifications to the plan itself.

Savings Plans, particularly EC2 Instance Savings Plans and Compute Savings Plans, are designed to provide flexibility in terms of OS usage. These plans automatically apply to your EC2 instance usage regardless of the operating system you're running.

This means you can freely switch between different operating systems (for example, from Linux to Windows or vice versa) on your EC2 instances without impacting the discounts provided by your Savings Plan. The plan will continue to apply to your usage as long as you're using the specified instance family in the chosen AWS Region (for EC2 Instance Savings Plans) or any applicable compute resource (for Compute Savings Plans).

So, if you've selected the "wrong" OS initially, you can change it on your instances without any concerns about your active Savings Plan. This flexibility allows you to adapt your infrastructure to changing needs without losing the cost benefits of your commitment.
Sources
Question about Shared Tenancy and OS release in light of EC2 Instance Savings Plan | AWS re:Post
EC2 Instance Savings Plan VS Flexible reserved instance | AWS re:Post