They will come out, you may need a pipe wrench.
They can be in really tight.
I messed up my wrist one year with pliers on a water heater.
I was told by the resident expert at Lowe's to replace the heat trap nipples in my water heater to increase water flow. My current htn's plastic center looks as though it's been melted and I can see why a small amount of water gets through.
I used channel locks pliars and tried to turn counter-clockwise but no budge... I don't want to ruin the threads in case they don't come out so I am waiting to see if someone can rescue me before I kill the thing....
help needed in SA
I just extract the plastic heat trap device from them. Otherwise, screw a fitting onto the threads so the nipple does not distort, then use a large wrench to unscrew the nipple. Replace it with a brass nipple, not a galvanized one.