You are asked for the root credentials when adding a host, but vCenter will then use the "vpxuser" user account to communicate with the host, i.e. changing the host's root password will not affect the host connection.
Moderator note: Moved to VMware vSphere™ Discussions
Normally I change directly from the esxi host (SSH, DCUI or WEB Client) or via powercli. I have never read the documentation of a command from vCenter to change the root password of ESXi hosts
Changing the root password of an ESXi host does not disconnect the ESXi host from the vCenter.
I have a test server to verify that. Once I changed the root password, vCenter lost it's connection to manage the host. However, once I changed the password back, the vCenter connection started working properly again.