What "nut" are you talking about. And what do you mean by "change the whole bottom end" ?Is there a way to get the chain OFF without opening the nut? I tried it and it always get stuck somewhere. I dont want to damage it. I just bought it new but I need to change the whole bottom end.
If you want to remove the chain all you need to do is take the master link off, disengage the clutch and pull the chain out of the engine, preferrably from the bottom.
Reinstalling the chain can be simplified by tying a piece of string to the end link, run the string through the clutch actuator cover and pull the chain through with it. It can be done without the string but often the chain will bunch up on the drive sprocket and jamb. If that happens don't try to force things with a screwdriver. You'll break something. You can also remove the spark plug and feed the chain around the drive sprocket with the clutch actuator cover removed.
Tom