You're safe to screw the top off if you can hold the shaft from turning. No nitrogen or oil should escape. There is a danger if the gas has got past the gas piston Oring but won't affect what you're trying to do. You'd know if that's happened because it'd be all gas springy with little damping. If you then wanted to completely dismantle it the shock body has to be drilled to let the gas out then drilled bigger to fit a special valve. That's the short version not including other things involved for a total dismantle.

Ok thanks.

Had to re-read what i wrote to be sure we we're just talking the end off the shaft to replace bump stop. All good.