Yes, you can update roles for already existing users. To update roles:
Login to console.plivo.com.
From your account menu, click Settings.
On the Accounts page, click Users.
On the Manage Users page, select the role that you wish to update for the specific user.
This will update the user’s user roles.
You can also remove an existing user from your account by clicking Remove.
You can remove or resend an invite to a new user by clicking Remove Invite or Resend respectively.
You cannot change the roles for the account’s owner.