Scale HAProxy for more than 64k websockets

Maybe Perlbal? This is a Perl-based load balancing solution. I have not worked with it myself but I've seen it referenced often and, since it's written in Perl, there's some chance it will work. But...

...Google suggests that Windows server already has a built-in network load balancing service, as discussed here and here.

I haven't tested this specific use case but haproxy does support a "hot reload":

2.4.1) Hot reconfiguration
--------------------------
The '-st' and '-sf' command line options are used to inform previously running
processes that a configuration is being reloaded. They will receive the SIGTTOU
signal to ask them to temporarily stop listening to the ports so that the new
process can grab them. If anything wrong happens, the new process will send
them a SIGTTIN to tell them to re-listen to the ports and continue their normal
work. Otherwise, it will either ask them to finish (-sf) their work then softly
exit, or immediately terminate (-st), breaking existing sessions. A typical use
of this allows a configuration reload without service interruption :

 # haproxy -p /var/run/haproxy.pid -sf $(cat /var/run/haproxy.pid)

If you have an init script to start and stop haproxy it likely supports the reload argument with a function like:

haproxy_reload()
{
    $HAPROXY -f "$CONFIG" -p $PIDFILE -D $EXTRAOPTS -sf $(cat $PIDFILE) \
        || return 2
    return 0
}