I doubt that you will be able to keep the same plates for a new registrant.  Plates in Switzerland are not really attached to the vehicle but rather to the registrant.

To terminate insurance, I'm pretty sure that the registration must be cancelled and the plates returned.

Hope this helps a little

Thanks for your reply.

In fact, the transfer of the plates is possible between family members, and in some business contexts:

https://www.ge.ch/document/conducteur-vehicules-formulaire-cession-exceptionnelle-plaques

And one of the insurers from whom I have sought a quote has included, helpfully, a draft letter of resiliation to send to the existing insurer, with most of the details already entered! I am tempted to go with that company for that reason alone.

That's interesting.  About 10 years ago I tried to transfer to my wife and they wouldn't let me.  It looks as if things have changed for the better :)

Well, it turns out that 'conjoint' (the word used on the form at the link above) does not -- in Geneva at least -- cover the same range of relationships that it does elsewhere in the world. As we are not married, with a ring and a certificate, we could not transfer the plates. Very annoyng, as we have to go through all the hoops and the expense of gathering documents to apply for a new parking macaron. I'm tempted to simply re-transfer the car registration back to my partner and to keep the old plates. It would be cheaper and less time-consuming than getting a new macaron.