I have about 37 vms running on 2 hosts. I need to change about 20 of them from 2 cores/4gb to 4 cores/8gb ram.
how do I know if the current cluster will allow this and how do I find out if I will be oversubscribing.
You can view the summary of the cluster by selecting it -> summary tab. There you will see the current admission control policy (type, and if admission control is enabled). You can then calculate based on the current host load if this would work, taking into account the admission control policy.
As Tom Said -- check for the load of the VM's in term of CPU and Memory
You can check this via the performance charts for a week and then estimate can your cluster take the load
Do not make the changes at once -- go step by step and verify the performance
