Safety first!
I had 2 glow plugs with swollen heads ounce but on 123s they are not such a problem.
Several people have had similar problems of getting them part way out; I do not recall that there is any special trick other than to carefully keep spraying (praying) and working them. The glow plug tips may be trapped by a carbon build up.
It might be a good Idea to ream out the carbon build up from the glow plug holes ounce they are out and for sure put some sort of Never-Seez type compound on the threads of the new glow plugs when they are installed.
You might want to check some more threads for your specific problem.

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84 300D, 82 Volvo 244Gl Diesel