Hello,
I think what you might be seeing is that powershell command passes, but when you see the properties of the VM, they will still be old.
My take is that you will need to use reconfig_VMTask to do this. For this you will need to power down VM in your script, set the parameters to reConfigVM_Task and power on the VM. I have used reConfigVM_Task and it works.
See this also - ReconfigVM_Task | VMware and Powershell
Thanks,
Pankaj