Can I change the image on RHEL HA instance?

Reserved Instances are not physical instances; instead, they are a billing discount applied to the running On-Demand Instances in your account. The On-Demand Instances must match certain specifications of the Reserved Instances to benefit from the billing discount. So, if you don't plan to change the instance type or family and are only changing the operating system, it typically won't affect your instance reservation. For additional information, please refer to the following link: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-reserved-instances.html.

You can't easily change the OS attribute of your Reserved Instance if it's a Standard RI type. But if your Reserved Instance is Convertible, you can use the exchange feature to replace your RI with another one with different OS.

Note that if you change OS on your running instance, but the OS remains the same on your RI, then the discount from the RI might not be applied to your running instance anymore (the attributes have to match).

If you have Standard RI, you can't change the OS, and you would likely need to wait until your RI expired, before changing the OS of your instance. If this is critical for you, I suggest reaching out to Support (open a support case), and ask Customer Service team if there are any options to replace your RI with the one with different OS. Most likely not, but I still recommend opening a support case and asking.