Sure, the REST API "update user" is an option. Though, I forget what happens when you do a tabcmd createuser or createsiteuser. Does it fail saying there is a duplicate or does it update the existing info? It's worth at least trying.
I was using tabcmd createsiteusers command to create the user and add to the default site. That command does not update the email for an existing user. The command that succeeded in changing the email is tabcmd createusers.