Repairing a new or old instrument always require a delicate touch. If possible, I always strive for the repair to be invisible. Even if the repair include cracks or parts needed to be changed.
The quality of the tone should always be the same, or better after the repair.
If possible, the varnish should be preserved, with improvements only if needed. Refinishing the varnish usually lowers the value of the instrument.
Trimming the tone and playability on a new or old instrument is most likely the hardest thing with building and maintaining an instrument.
After many years of experience, I have precised the art of adjusting bridges, soundposts, and other parts on an instrument precisely according to my customers wishes.