If at least 1 other person RSVP's for an event, and you change your status to no, there exists no possibility to RSVP again and change to yes.

The reason I say 1 other person is because if you are the only one, it gives you the Be the first to RSVP option after you say no, so you can still change back to a yes at that point.

Please tell me how to fix as I bought the pro version specifically for RSVP and a lot of my site's members travel a lot and they need to be able to change their RSVP without worrying about being locked out of it.

It sounds like a bug. I'll take a look at the problem. Are you using the 2.0.10 version of the plugin?

yep 2.0.10.2.

Fixed it with a hack!

You had this in the code:
switch($rsvp->status){
case "1":
$htmlstr .= "<br/><a href='$rsvpurl'>" . JText::_("ZC_UNRSVP_TO_THIS_EVENT") . "</a>";
break;
case "2":
$htmlstr .= "<br/><a href='$rsvpurl'>" . JText::_("ZC_TUNRSVP_TO_THIS_EVENT") . "</a>";
break;


I just added
case "0":
$htmlstr .= "<br/><a href='$rsvpurl'>" . JText::_("ZC_RSVP_TO_THIS_EVENT") . "</a>";
break;

That way it is checking for the selection to be No, and gives you the message "You have not RSVP'd to this event. Click here to RSVP."

Thanks for finding the problem. I will include this in the main release so you won't have to hack it every time you upgrade.