Can we remove a security group from an running EC2 instance?

I have a running EC2 instance with few security groups added. What I want to do is to remove one of the security group and add a new one.

I think there will be no problem in adding a new security group. But is it possible to remove a security group without terminating existing ec2 instance and creating a new one from the ami ?

yes, it is possible both to add and remove security group. In AWS web console go to EC2 Instances page, right click on instance you want to change -> Security -> Change Security Group You can easily check that it is done without terminating or even stopping the instance.