They make a Glow Plug Reamer to clean out the Carbon from the Holes and it will not damage anything.
You can buy one, make one or if you are in a hurry there are some alternateive methods to cleaning the hole.
A Factory Made Glow Plug Reamer has a 7mm Reamer on the end of it. So anything that size or smaller ought to be safe.
Also Autolite Glow Plugs are simply awful; after 6 months use I barely got 2 of them out of my Cylinder Head as the Glow Plug Tips had swollen at the end.
Monark Glow Plugs are not as bad but also do not last (they lasted about 1-1/2 years in my car).
Bosch or Beru Glow Plugs are the best with the Bosch ones being the easiest to find and the least expensive of the 2
When you install the new Glow Plugs put some Never-seez type compound on the threads.
Also if you have to remove the rest use something with 6 points on it so you will not round off the Hexes.
Putting the PB Blaster in the Freezer was inovative!
See what you can find below concerning cleaning the Glow Plug Holes.
DIY Repair Links
http://www.peachparts.com/shopforum/forumdisplay.php?f=82
http://www.peachparts.com/Wikka/DoItYourSelf