The actual changing of the account is done from the Windows Services panel (the two gears icon), but the other piece of this is to give that domain account full control (or as close to it as possible) over the same GeoEvent folders on the hard drive.
Thanks once again. I am not sure if it is related to this (hence my post) but we are getting windows log errors after we initially changed the GE service account to a domain account:
The machine-default permission settings do not grant Local Activation permission for the COM Server application with CLSID
to the user DOMAIN\svc_aztst_arcgis SID (S-1-5-21-6458xxxxx-xxxxxxx-xxxx-xxxx7632) from address LocalHost (Using LRPC) running in the application container Unavailable SID (Unavailable). This security permission can be modified using the Component Services administrative tool.
These start shortly after 12am (guessing it is just after the service refreshes).