​release announcement
For win32 this should just be a matter of rebuilding from source, on Linux/Ubuntu we will probably wait for packages to show up ​here

I just installed xpra 0.4.0, and, to get my session upgraded in-place, did a kill of the winswitch_daemon, and reconnected.

winswitch tried to restart xpra, I believe:

26717 ?        S     22:55 /usr/bin/python /usr/bin/xpra --xvfb=/usr/bin/
26718 ?        S      4:55  \_ /usr/bin/Xvfb-for-Xpra-:67 +extension Comp
26786 ?        S      0:00 python /usr/lib/winswitch/delayed_start /home/
26787 ?        S      0:00  \_ /bin/sh -c xchat
26788 ?        Sl    35:13      \_ xchat
13327 ?        S      0:00 /usr/bin/python /usr/bin/winswitch_server --da
13371 ?        D      0:00 /usr/bin/python /usr/bin/xpra --bind-tcp=0.0.0

But something went wrong, and the new session was killed - *including* Xvfb and my active xchat.

The logfile has these entries:

[II] 2012/23/06 22:53:33 XpraServerUtil.start_daemon(ServerSession(:67 - xpra - available),
[II] 2012/23/06 22:53:34 virt_server_daemonizer.grab_daemon_pid(['/.../virt_server_daemonizer', ...
 '--', '/usr/bin/xpra', ... 'start', ':67', '--use-display'
[II] 2012/23/06 22:53:34 XpraServerUtil.session_process_started(13371,ServerSession(:67 - xpra - available))
[II] 2012/23/06 22:53:34 XpraServerUtil.do_watch_session_log(...
[II] 2012/23/06 22:53:34 WinSwitchServer.publish() mDNS_publish=True
[II] 2012/23/06 22:53:34 AvahiPublisher.__init__(Window Switch for ...
[EE] 2012/23/06 22:53:34 AvahiPublishers.start() error on publisher AvahiPublisher(Window Switch for ...
[ee] 2012/23/06 22:53:35 Traceback (most recent call last):
[ee] 2012/23/06 22:53:35   File "/usr/lib/pymodules/python2.7/winswitch/net/avahi_publisher.py", line 64, in start
[ee] 2012/23/06 22:53:35     publisher.start()
[ee] 2012/23/06 22:53:35   File "/usr/lib/pymodules/python2.7/winswitch/net/avahi_publisher.py", line 105, in start
[ee] 2012/23/06 22:53:35     bus = dbus.SystemBus()
[ee] 2012/23/06 22:53:35   File "/usr/lib/python2.7/dist-packages/dbus/_dbus.py", line 194, in __new__
[ee] 2012/23/06 22:53:35     private=private)
[ee] 2012/23/06 22:53:35   File "/usr/lib/python2.7/dist-packages/dbus/_dbus.py", line 100, in __new__
[ee] 2012/23/06 22:53:35     bus = BusConnection.__new__(subclass, bus_type, mainloop=mainloop)
[ee] 2012/23/06 22:53:35   File "/usr/lib/python2.7/dist-packages/dbus/bus.py", line 122, in __new__
[ee] 2012/23/06 22:53:35     bus = cls._new_for_bus(address_or_type, mainloop=mainloop)
[ee] 2012/23/06 22:53:35 DBusException: org.freedesktop.DBus.Error.NoServer:
  Failed to connect to socket /var/run/dbus/system_bus_socket: Connection refused
[II] 2012/23/06 22:53:36 XpraServerUtil.detect_sessions() found: [('DEAD', ':67')]
[II] 2012/23/06 22:53:36 ServerSession.close() self=ServerSession(:67 - xpra - closed), killing ...

Perhaps there's a way to get the new xpra running on a different port, and only grab the Xvfb and kill the old xpra when everything else is ready?

See ​https://xpra.org/trac/ticket/43#comment:23

Probably need to duplicate the workarounds to get Ubuntu to stop doing crazy things.