If you can get them out of the doors, put them in a little cup of penetrating oil overnight and then see if the key will work them loose. Usually they are seized from minor corrosion and dirt, not so much from physically welding themselves together on the inside. Sometimes a good soak will loosen up any dirt that's in there.

I don't know about disassembly, but it sounds like something I wouldn't try. I've never split a lock cylinder apart since they are so easily replaced.
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1992 940 wagon, low miles
as well as others.