Luc Dekens has a Powershell script that modifies the ESXi host root password. This is non-disruptive.
The original script and info is here: http://www.lucd.info/2012/01/15/change-theroot-password-in-hosts-and-host-profiles/
I have seen on the VMTN people stating the script works for vSphere 5.1 and 5.5.
When I was testing this in my lab environment which is vSphere 6 the script kept throwing errors.
Below is the script modified from the original and it works on my vSphere 6 hosts.
All credit for the original script to Luc Dekens @LucD22
Connect-VIServer -Server labvc02.lab.local -User lab\bob -password xxxxxxx
A note that in vSphere 6 the password policy complexity is now stricter.
Musings from a past life 