How can I move EC2 instances to a different subnet?

I have a VPC set up in AWS. we have created 2 subnets (A/24 and B/24), straight using all the IP having no IP's/Subnets for further usage.

We find that the b/24 is not totally utilized and we are wasting the Ip address in that.

Now we want to reduce the subnet size and move the instances from B/24 to A/24. and create 4 smaller subnets from B/24 and of /28 range and move back the servers to one of the smaller subnets created.

Please advise what are the suggested best practices we can apply in a way that we should be able to keep our available servers in B/24 and create multiple subnets as well.