The detailed analysis is as follows:
Rebuild / Generalüberholung
Scope of Work: This represents a large-scale, thorough reconstruction of the piano. It typically includes replacing the pin block, restringing, repairing the soundboard, and installing new hammers and all worn action parts.

Core Objective & Principle: The main goal is to restore the instrument’s sound, mechanical function, and appearance to “as close to new” condition, optimizing its performance. To achieve high-quality playing results, the use of non-original, high-quality third-party replacement parts is permitted.
Restore / Restaurierung
Scope of Work & Principle: The standard for restoration is even more rigorous than rebuilding. Its core definition is “to return the piano as close as possible to what it was when it was new.”
Core Difference: Restoration not only requires the piano to look and feel new but also demands absolute fidelity to its historical authenticity, strictly using original factory parts and original manufacturing techniques.
Most Illustrative Case Comparison (using Steinway pianos as an example):
The essential difference is this: If you are simply rebuilding a Steinway piano – aiming to make it play and sound well – you may use brand new hammers from any other reputable brand. However, if you are restoring a Steinway piano, you are required to use only Steinway’s original factory hammers.
(Note: In addition to the two levels above, there is also the most basic level of Repair (German: Reparatur), which refers only to fixing damaged or worn individual components without extensive disassembly or replacement.)
Summary:
Generalüberholung (rebuilding/reconstruction) focuses on the revival of performance and practicality, allowing for flexible use of modern, high-quality components. Restaurierung (restoration), in contrast, focuses on preserving the original integrity of the instrument as an artifact, pursuing the highest degree of historical authenticity.
PS: Strictly speaking, true restoration is akin to archaeological conservation, where no modern materials or replacement parts may be used at all.