Did you put cloth under the pliers when you unscrewed? That will help not mar the screws.
I had DH take the 503A in a cabinet outside this summer so I could sew.
Nothing's impossible with enough muscles. Preferably his, not mine in that case. :P
I've also found that with the curve of the screws, I like to use the rounded part of the pliers, not the very tip,.. tends to spread the force out some. Just a thought.