A warm engine should make spark plug removal easier, so its good to try after a drive when its still warm. Perhaps try tightening back down and then loosening again, repeatedly, to see if you can make progress with out using much force on them. I usually feel a bit of resistance or "crunchiness" as you unscrew them, but never enough where I thought I'd strip threads.
Definitely back down if it feels wrong, don't force it. They can be a struggle to get out sometimes.
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20mm OD x 12mm ID x 4mm W : Viton O-Ring
3/4" OD x 1/2"ID x 1/16" Thick : Thrust Bearing/Washer