Groups on Airwave comes to picture only post the cluster is made. Moving an IAP from one group to another will not make any change to cluster on the physical side.
For example lets say you have added a new VC-A to group A on Airwave which currently dont have any IAPs under it. You already have VC-B with 50 IAPs in group B on Airwave.
In this case if we move 25 IAPs which are under VC-B(groupB) to Group A where we have VC-A, this is not going to change the cluster for these IAPs. It will still be seeing VC-B as there master.
If we need to divide the IAPs and form 2 cluster that needs to be done directly from IAP side, post that you can create 2 groups on AIrwave and add the VC to both group. So IAPs under VC-A will be in one group and VC-B will be in the second group on Airwave.
Post this you can manage the separate cluster from Airwave.
Note: Groups on AIrwave doesnt play any role in changing or creating new cliuster. New cluster needs to be created on IAP side itself prior to adding device on Airwave.
If you are still confused i would suggest opening a TAC case they can help you explain the same over a call or session.