Downgrading

I have had my phone g900a, rooted since 4.4.2. My current OC4 is also rooted using FlashFire. I read that if the phone was already bought with the current system, it can't be downloaded, but mine wasn't. How do I go about doing this? I'd rather go back to original stock and upgrade keeping it unrooted.
If you can, please share the thread that shows me how.