1990 Pontiac Sunbird 2.0L difficulty removing the old plugs

1990 Pontiac Sunbird 2.0L mileage: 132,000. I changed the spark plugs at 55,000 miles and had difficulty removing the old plugs. I think carbon built up on the internal threads that exposed to the cylinder made removal difficult. On one plug the threads in the aluminum head were somewhat damaged but I was able to install the new plug. The car is running rough and I expect the plugs are the problem but I am afraid of making things worse so I am looking for advice before attempting to change the plugs. Should I change the plugs? Are there some tricks to prevent thread damage?