There is a fine line between a piano restoration and rebuilding. In a way, the distinction is artificial because functionally restoring the original condition of a piano can require as much rebuilding, i.e. replacement of parts and structural incisions, as in a true rebuilding.
The traditional view of piano restoration is that original parts are preserved whenever possible. The parts may be cleaned, repaired, and made fully functional, but as long as they are not replaced, the original value and character of the instrument is thought to be preserved. This is often an appropriate goal, though it can be argued that restoring original parts can't really restore them functionally because their original physical and chemical properties have changed—hammer heads lost their resilience, shanks are more brittle, all felts have hardened and may be contaminated with lubricants. Few technicians would argue that strings shouldn't be replaced as part of a restoration. New strings will be simply more reliable and sound better. Why then whouldn't we replace action parts with dimensionally accurate replacements? Wouldn't that match the "original condition" more closely than preserving the original parts?
Rebuilders see any modern piano (American piano made after c. 1880, European piano made after c. 1910) as an opportunity to create a functional equivalent of a brand new instrument or to reproduce a particular historical target. The attitude is to do whatever it takes to make the instrument satisfy pianistic expectations of the target era. The rebuilder's approach is less constrained—he or she can be more open to trying different combinations of parts to achieve a particular musical goal.
My focus in rebuilding modern and pre-modern grands is to make the piano satisfy your musical goals. I can redesign an action, rescale the piano, and change its soundboard characteristics, or simply restore the original design with or without modernizing the touch response.