New SEP installations not pulling updates from GUP

I've successfully configured a GUP on our local network and accoring to the "debug.log" file I can see that existing SEP clients are now pulling their updates from the GUP but for some reason for new installations the virus definitions continues to be downloaded from Symantec LiveUpdate's web site. Is there a way to make new installs pull the updates from the GUP also? We're running version 12.1.4. Thanks!