I had the same issue here when I first got my 850.

The others are right, avoid the breaker bar.. very risky to get too aggressive.
If you can budge the plug, work it back and forth to help a small amount
of Mystery Oil (my favorite) penetrate better. Don't over-do it with the
penetrating oil, you only need a little for the threads, and more just makes a mess.

In my case, I needed to cleanup/retap the sparkplug holes, which brings up
another problem of not wanting metal chips in the cylinder/valves etc.
I was lucky to borrow a real tap from a local garage,
(there's another thread cleaner gizmo cheapo tool that I saw at another garage, that is a piece of junk.)
I put a lot of wheel brg. grease on the tap to catch the chips, and it worked OK.
If there were any chips in there, I suppose they were small enough to
blow out the exhaust.

I used Never-Seez on the new plugs, and that worked fine.
I needed to retap the threads for the plug cover too, those steel Torx head
screws were also seized up in the aluminum head, they need Never-Seez too.

Later I used that same tap to cleanup the threads for the O2 sensors, so that's
a good tool to have!

Good luck, Bill

PS: While you have the Mystery oil out, put some on the thermostat cover screws,
there's another problem location with steel screws in aluminum.