Dear Jean-Pierre, all,
Thank you for your question, and apologies for the long delay of this reply; This is the evidence that your question is relevant, because it triggered some additional discussion.
In short, we can say that Members can start using WIGOS IDs anytime, as long as they follow the guidance, particularly the (minimum) three months advanced notification, before distributing new reports that should include both traditional station ID and WIGOS ID.
For additional guidance please refer to to the WMO circular letter on "Reporting of WIGOS Station Identifier in BUFR/CREX messages - guidelines for introducing the WIGOS Station Identifier in FM 94 BUFR and FM 95 CREX messages", which is here attached.
Luis (in consultation with Steve)