The new Citrix Online Plugin 12.1 was released last week and the new interface looks really good. Hopefully we now got a working bug free client. In my opinion there have been way to many bugs after Citrix went from the old style Citrix XenApp Client to the new Citrix Online Plugin supporting the HDX features.
I highly recommend you taking a look at Citrix Merchandising server to manage your Citrix clients, but it is pretty easy to upgrade your existing clients as well. This could be done with Microsoft SCCM or by using a startup script in Active Directory. Citrix have created a script that propably uninstall existing clients and install the new version. I have used this script at many clients and it simply works.
The unattended script can be found here : Sample Script to Install CitrixOnlinePluginFull.exe