Is it possible to change from restricted B permit (non-eu) to Carte de legitimation on a job offer from an International organization ?

You will get a carte de legitimation but it's valid only from your first day of work in an international organisation.

Absolutely possible; done so myself. You won't have to leave the country, either - notify the HR department of your new employer, and they will return your permit to the Swiss authorities.

Hi pickles, what do you mean by restricted B permit? Is it tied to your employer?

The reason i asked is Carte de Legimation is only valid as long as your employed in an International Organization, once your contract expires you are only give 1 month to stay here in Suisse (for non-EU nationals). If you are signing up for a consultancy or temporary contract better check if you can keep your permit B.