Yeah it is hard to rotate. Jack up the car nice and high so you have some room to work (use jackstands) get a big screwdriver, carefully lever it around using the holes in the housing. I don't think you can hurt the converter but you don't want to break the housing. Do it in little increments, and you have to switch from one side of the housing to the other sometimes (but keep turning the same way) and do it with the fluid warm. That's where most of the fluid is, not even worth doing if you don't do the converter too. Parking brake shouldn't affect it...

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'84 300SD veg-oil bliss
'83 300SD veg-oil beater