downgrade from clean install of windows 10 v1709 to v1607?

Hi, the only way to do that is with the use of 3rd party software.
The most cost effective would be
a. Create a disk image of what you have
b. Clean install the new OS
c. Install the appropriate Easeus or Laplink product (one time cost)
d. Let the software 'install' your progs from the disk image.
These work well- I've had 96%+ success rate with Laplink with hundreds of programs installed. Good on configuration and settings too.

It's a question of your time to reinstall and configure vs money.

Note- from 1709 the next step back would be the CU at 1703.

Then if you install 1607 you would have to disable Windows Update - or with Pro you could defer the upgrade for a long time.

Note too had you followed our so often repeated recommendation to use disk imaging routinely, and you had an image of your system with 1607 installed, you could have restored that in under an hour.
E.g. Macrium Reflect (free).