Downgrade AT&T Galaxy S5 to root?

Hello everyone! I just got a Galaxy S5 from my mom (she got the S6 Edge for her birthday) and I wanted to root it but I found out that the bootloader on the Lollipop versions of AT&T S5s are locked and you cant root. This really sucks because my other phone just broke a few days ago (no, I don't break my phones every few days, it was just a coincidence that it broke right before my mom's birthday) and It couldn't root either no matter what provider or version. I heard that with the S5, you could root before Lollipop so maybe I could downgrade to KitKat and root. Maybe? If that will work, can I upgrade back to Lollipop, will the root still work when I go back to Lollipop, and how do I downgrade the phone? Thanks to everyone who actually took the time to read All of this and who help me!

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