Julles, we have painful memories of that sort of thing. We have an old (1860) French armoire that was "smuggled" into the USA by ancestors during the war, that was worth a small fortune. My MIL "refinished" it and it lost 80% of it's value.
However, financial value isn't everything. We really do enjoy the beauty of the piece, even though it was refinished. I love the wood and how pretty it looks refinished with boiled linseed oil. It probably looked a lot worse covered in black grime. We'll take it either way... but it really looks wonderful with the refinishing
I guess it's about money, beauty, or true conservation. True conservation is a labor of love.
The site that Esther posted should get you going, it's a great learning site.