I have a problem. I printed earlier with the 0.2 mm nozzle but none of the prints really went well. So I wanted to switch back to the 0.4 mm nozzle. It seems the nozzle is pretty much stuck. I can't unscrew it. (I don't want to use too much force and maybe damage the extruder.)
I've had that exact problem, I guess your nozzle, like me, leaked some plastic, you will have to put more energy (I had to force a lot to remove my 0.2 nozzle).
Okay guys, I managed to remove the nozzle by heating the extruder while unscrewing it.
However the plastic part at the top got unscrewed as well and was left in the extruder. I had to unscrew some parts from the extruder to take it out of the top.
(some of the "wool"(?) near the heating element came out as well. I pushed it back though)
It seems the PTFE tube melted/clogged and it got stuck. (also notice the dark color of the screw)
The thread looks okay, you can use it as backup when your real backup nozzles are used.
I used to soak nozzles in acetone when they got stuck with ABS. I don't know what to soak in with PLA. Remove PEK, soak, clean bore and clean thread so you can reuse the nozzle.