I'm with CJO and Silvers on this one. Assuming that one is starting with a well worn / abused sterlingworth or other common gun, which has no real collector grade value, upgrading said piece can be a wonderful option. In fact some will go so far as to select a gun that would otherwise be parted out to use as a platform for such a project, thus saving the gun from the scrap heap.
As long as the SN remains all is well with the world, a gun owner gets his own expression of what a custom gun should be (very satisfying for the owner on personal level), and someone in the trade gets a commission.
On a personal level I would never alter a collector grade gun. I have however customized well worn sterlingworths.