So, all this talk about having someone build a custom guitar for you, what about having alterations made on one that you already own?

Personally, trying to make major adjustments (like changing the neck/making the neck thinner or trying to make alterations to the headstock if that's even possible) makes me a bit woozy...but I do like the idea of changing the tuners, buying better bridge pins (I've heard this can improve sound?), adding a "strap lock system" and changing the pickguard...if you're pleased with the sound of the guitar and you just want to adjust things to make the guitar look better and easier to play...it seems like the smart thing to do, rather than trying to buy a different guitar.

    So, where would be a good place to find items like this and what all sort of adjustments can be made?
Anyone ever tried using specifications on a higher end guitar to make adjustments one that they own?