Easy fix!
Ok first off before you remove the nut just loosen it up maybe one turn then use a brass hammer or a 2lb hammer and a piece of wood and strike the nut just to see if it is gonna come out and you won't damage the threads. 2nd if it doesn't move the best thing to use is a 3/4 impact wrench on the opposite side and see if it will spin if it spins it will come out! If you don't have an impact use at least a 1/2 drive breaker bar and try to free it. And if that doesn't work you are gonna have to use a firewrench (torch). The bolt isn't stuck in the swingarm it is stuck in the frame. So just heat the frame around the bolt and don't over due it, It doesn't take much heat to expand the metal enough to free the bolt. When reinstalling the bolt the best mix I have seen is a synthetic greased mixed with anti-seeze. I wouldn't go through the hassle of removing the bolt just to lube it once a month if it really concerns you I would just loosen the nut and spin it a few times just to make sure you will be able to remove it when you need to.
I have only seen one instance where the bolt wouldn't come off and that was a 1985 tri-z 250 that had the original swingarm bearings in it. We tried removing the swingarm bolt in 1998 needless to say it would not come out ended up cutting it off and pressing what was left out of the engine with a 40 ton press and alot of heat it finally came out. But you wont have that problem with a 400ex unless you park it in a swamp for a few month's.
Previously owned bikes:81-yz80,82-ty80, 85-tri moto 200, 84-rm125, 85 xr200, 84 rm500, 87 rm125, 83 honda big red, 88-250r, 89-250 quadracer, 90- 500 quadzilla,86 cr500, 91-250 quadracer, 86 tri-z, 89 xt600, 94 fzr600, 95 timberwolf, 93-banshee, 88- 265R, 01-250ex(wifes). 2001 400ex, 440 kit, 03 ds650, 00 big bear 4X4, 01,xr200r, 1995 wr250z, 2002 yz250 2smoker, 2002 husky wr360, 2004 400ex