WHAT IS A REFIT, AND WHY YOU MIGHT WANT ONE.
In a well maintained, older boat that has never had a full stem-to-stern refit all of the systems are of different ages. Some might be original, some might have been replaced last month. The issue is that some systems on the boat are always approaching the end of their function lifespan, so keeping systems running is a constant game of whack-a-mole as the older systems on the boat fail and need repairs or replacement.
The strategy for a refit is to reset this clock for as many systems as possible, so they are all new, or like new, and an extended period of time can be expected without major repairs.
There are various levels of refit that can be implemented depending on the clients needs, budget, schedule, and cruising plans. At the highest level, many of the systems on the boat are simply replaced with the new. This has the advantage that the newest technology upgrades are available, but is the most expensive, and requires careful shakedown to be sure that the new systems all preform as required. A simpler, less expensive refit would involve replacing some systems, but rebuilding or repairing others.