Professionally performed refits
Even if your yacht was built at the time entirely according to your specific wishes, over time the desire may arise to update the technology or adapt the layout to changed use; perhaps the children have not sailed with you for a long time, or the arrival of a grandchild is reason to create extra sleeping capacity.
It is often thought that such a refit never pays for itself, but this is based on a common misunderstanding; a professionally executed refit translates into a significantly higher residual value when sold, because a contemporary good yacht is simply more interesting than a ship with a dated interior or very outdated technology.