There is no way to change the registration status of an existing registrant. It would need to be deleted and added again with the correct registration state, or in this case the alternate host+panelist. Please see example below using createMeetingAttendee API.
<?xml version="1.0" encoding="UTF-8"?>
<serv:message xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<header>
<securityContext>
<webExID>USERNAME</webExID>
<password>*********</password>
<siteName>SITENAME</siteName>
</securityContext>
</header>
<body>
<bodyContent xsi:type="java:com.webex.service.binding.attendee.CreateMeetingAttendee">
<person>
<name>Panelist 1</name>
<email>Panelist1@example.com</email>
<type>PANELIST</type>
</person>
<role>HOST</role>
<emailInvitations>TRUE</emailInvitations>
<sessionKey>1234567890</sessionKey>
</bodyContent>
</body>
</serv:message>