If its doesn't sound great why buy it?
It doesn't make sense to me to spend money devaluing your guitar.
Conversely, if it does sound great, why sell it? And that being the case, why worry about the value? Ultimately a guitar's value is irrelevant unless/until you decide to sell it. They're supposed to be musical instruments after all, not investments.
I must agree, though, that I personally
would be reluctant to change anything on a vintage guitar - not because of the value, but simply because I think it's somehow "wrong" to mess with something that's been left
unchanged for a long time. Which is one of the reasons I don't buy vintage guitars. Then again,
some of our new guitars are the vintage guitars of the future, so should we be concerned about devaluing them too?
But we're only talking about changing pickups, and this
is the Bare Knuckle Pickups forum, so I guess most of us aren't opposed to pickup changes. Lots of guitars feel and play great, but don't
sound as good as they might, and a pickup change is a very good non-destructive way of solving that problem.