Never done it before, but I think you have to stop your cluster, create a snapshot and an AMI image of this snap based on the actual NN, deploy the larger machine using that AMI or dettach the disk from the old host, attach to the new, boot the new VM host using the disk from the old, and increase the NN heapsize on Ambari.
Creating a snap and AMI image is a good idea anyway to have a backup if something goes wrong.
Hi Pavan, Did you manage to change the Instance Type? Even we are upgrading it from m4.4xlarge to m4.10xlarge, let me know if you have faced any issues post doing this. If it was successful can you post the brief steps. Thanks.