Dear George,
Thank you for your message. Please see below a few comments.
It is acceptable to provide additional WSIs to stations that are already registered in OSCAR/Surface with a WSI assigned by WMO, e.g. during migration from Vol.A as those with Issuer of Identifier (2nd block) in the range 20xxx.
However, to minimize impacts for users it is advisable to avoid additional WSIs when/if possible.
On the other hand, to allow historical tracking of station records in OSCAR/Surface, WSIs shall not be deleted, only new WSIs can be added and be flagged with the primary key.
Finally, regarding your BUFR related questions, I'll consult with colleagues on the WIS (WMO Information System) branch in charge of coding and data exchange activities.
Kind regards,
Luis