Vintage guitars in their original state do command higher prices. As with any antique, condition is everything. That's not to say changing a cracked or deformed pickguard is a bad thing. First and foremost, guitars make music and if a bad pickguard affected the playability of the instrument, that would devalue it also. What's important when changing or replacing something on a vintage instrument, is that it is the correct replacement. Keeping a guitar as close to it's original condition with the exact replacement is paramount to not letting it fall into "modified" status. this includes changing tuners, especially. Strings, of course do not count.

The less original an antique is, the less it's value.