I deliver auto parts. Garages are getting one plug all the time. You getting a misfire code for one cylinder? Sure why not. Personally I would just say screw it, replace them all, and call it a day. But then it would also depend on how hard it is to get at all the plugs.

Don't change all of them unless you like wasting money.

Excellent choice in a single cylinder engine but you haven't accomplished anything in a four, six or eight cylinder engine. If the rest of the spark plug gaps have burned .020 larger than factory recommendations it's s great way to burn-up the ignition coils.

all ways replace all plugs at the same time .all set plugs to the book if old plugs need cleaning and if they are ok clean then and keep them as spares when you need them

Sure! If only one is dirty or broken, why not. But while you are at it you should check the others too. Chances are all or none will need replacing, not just one.