restringing of my mikimotos

by Nikki
(Panama City FL)

I have a strand of 40+ year old, hand strung mikimoto pearls, perfectly matched. I took them to the local mikimoto dealer to be restrung, since they never have been, and they refused after looking at them, citing insurance issues because they are irreplaceable.

They told me I had to send them back to Mikimoto in Japan for
restringing. So, 2 questions, if I send them back, how much should I insure them for and should I even at this point have them restrung, or will it depreciate the value of them?
Thank you so much for your time!