When i change plugs in these units, i will loosen them and feel for resistance when trying to back them. If any is felt, i will work the plug back and forth several times splitting the difference until plug feels loose. Using shop cloth or paper towels i will encase the insulators of the plugs and pack loosely against head. Then i liberally apply penetrant oil to paper towel/cloth until oil cannot be absorbed by cloth/towel anymore. Tell the customer i will be back the next day to remove plugs. If customer is bent on unit to be repaired pronto i will try to work them with some kind of wonder oil while attempting to wrench them out. But if plugs are stubborn and know it is pulling threads, i will simply grab my impact wrench, remove the plugs with the threads and install coils. I let the person paying for repairs decide if another day is worth it. Most of the time there is not enough threads left to chase and once they start pulling it compounds the further they back out. Thread chasing can be gotten away with when using the longer shanked plugs but forget it on the new designed plugs as you will not get torque specs most of the time and you will leave the next guy ( which is probably going to be you anyway) having to install coils.