When a Koetsu is sent for rebuilding, all the elements in the cartridge are replaced - only the cartridge body is retained. This might be seen as wasteful but there is a strong reason for it.
A phono cartridge's performance does not merely depend on the condition of the cantilever or stylus. A cartridge with a brand new stylus is not going to do the music any good if the suspension is weak or if the coils are oxidised. This is akin to replacing the engine oil in your car while retaining the old spark plugs and oil filter. The car will still work, but it's performance will be terribly off-spec and suboptimal.
Replacing all the elements in your Koetsu phono cartridge will give it a new lease of life. It will be equivalent to getting a brand new cartridge. In fact, as the Rosewood body ages, it will add considerable value to the sound (which is why the Rosewood Signature Platinum [aka Pro IV] uses Rosewood which has been allowed to age naturally).
(TIP: Want a great sounding Koetsu at a lower cost? Get an old Rosewood Standard or Signature and have it rebuilt!)
Rebuilding your cartridge ensures that it will perform at the level which you've come to expect from your Koetsu cartridge.
For the more demanding Koetsu owner, Koetsu Japan is able to rebuild higher spec innards into their current cartridges. For example, upgrading the existing cores with the Platinum-alloy set.
Another upgrade is the Diamond cantilever option. As the name suggests, the stylus/cantilever assembly is machined out of a solid piece of diamond. This 'monocoque' structure effectively removes the interface issues associated with regular stylus/cantilever assemblies.
This is how Koetsu preserves your investment...
