Hello,
As Brian and Mithun said,it's recommend to use same version with manager and clients,however it's very difficult to compare with SEP 11* and 12.1* versions.
For my lots of field experiences i couldn't recommend you to don't upgrade your 11.x clients and also you're thinking about to not upgrading GUPs. In 12.1 lots of improvements for update technologies and GUPs.
So i heavily suggest you to upgrade all your clients to 12.1 and most off all your GUPs.
By the way if you're thinking to upgrade 12.1 in a couple of months,i also suggest you to wait for SEP 12.1.2, there will be lots of improvments,features and also a couple of new things in GUP technology also.
Regards,
Oykun