Downgrade after upgrading to Android 6 - knox tripping? G900F

Hello!

Sadly my G5 ist broken and I have to send it to a reprair center.
The center has the habit to upgrade any phone they receive to the newest firmware

My G5 was delivered with Lollipop - I downgraded to 4 to use Towelroot.

So my question - if they force me to Android 6, is it possible to downgrade without tripping knox?

It's a G900F btw :)

Thanks in advance!

Best Greetings!

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