So I stupidly set the refueling station for trains to be "REFUEL" but now I'm building a competent refueling setup with stations named "Refuel L" "Refuel LL" etc. Problem is, I can't change the refueling station setting so the trains are now trying to go to "REFUEL" which doesn't exist anymore. I tried changing the station to "Refuel" "Refuel L" and "" but it's not getting saved, whenever I clock the set for all trains button and exit the GUI and open it again it's back to "REFUEL." This also happens if I try to change any of the fuel thresholds.

Any help would be appreciated.

EDIT -- I'm running v. 4.0.9 on 0.17 btw

I hope it's a user error due to bad/old GUI design.
From your description it sounds like you change the Refuel station name, click "Set Refuel" and then close the gui by pressing E or Esc?
You have to click save for the changes to take effect.

It works for me with 5.1.1 on Factorio 1.1.15 but since there haven't been any changes in the last 17 months it shouldn't matter.