might try this,
KY redneck solution
Secure a good pipe wrench with sharpe teeth in the jaws so it will hold.
have a buddy jack the front of the tractor up place the pipe wrench on the bolt head with cheater bar extention against the ground.
now have your buddy slowly lower the weight of the tractor down,
this process will use the weight of the tractor to turn the bolt.
If this does not break the bolt loose you are up a creek
Probably have to drill out the bolt and use sometype retread device, but this would be a last resort as now you would have metal chips in the oil pan.
before you break something or get hurt or drill out the oil drain plug.
Question: can you stick a small rubber hose down the oil fill pipe and suck the old oil out? if so this might be an option.
if the oil drain plug is this tight, no telling how long its been since the oil has been changed and no telling what you will find in the bottom of the oil pan.
also, you may have the same problem with the oil filter canister, if its been that long since an oil change.
let us know what happens.
note: if you do have to take the oil pan off you may want to take it to a machine shop and have them get the oil plug out to save the oil pan, if you ruin the oil pan you are talking some bucks.