If you shut the original instance down (not terminate!) then the EBS volume will no longer be attached to an instance; you can attach it to your new instance then.
However: That will give you two volumes; a "new" root volume and the original root volume attached to a different mount point/drive letter.
Note that in some circumstances it will be possible just to change the instance type and boot on the original root volume: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-instance-resize.html
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