Have you tried uninstalling and reinstalling the drivers? Big OS updates usually require that for everything to work as expected with Creative's software.
I have multichannel audio working with an old X-Fi card on 1803, so I would expect the newer Sound Blaster Z to still be working.
A Pro license lets you defer feature updates for ~15 months on Windows 10.
Security updates are far shorter though, due to their importance. Being able to ignore updates indefinitely is not a good thing.
Manufacturers shouldn't have to update their drivers for every new OS update.
The driver for my X-Fi is from 2014, and the driver for my DAC is from 2012. Both are working just fine on 1803.
This is on Creative rather than Microsoft really. Whatever they're doing doesn't always seem to migrate across major OS updates.
I run my system through the HDMI in my video card to an AV receiver, and I'm getting 7.1 audio just fine. Some times when I get a driver update, I'll have to reenable 7.1 audio though, which seems to have been made more difficult with this update. I'm not sure if you're experiencing a similar issue.
Try right-clicking the speaker icon in the taskbar, select Speaker Setup -> Advanced setup. Click on your audio device, then hit the Configure button. On the next screen, pick 5.1 or whatever you want to use, then hit Test to change the signal and make sure it's working.