I added a MEI to an existing CFII ASE on Jan 8th. I received two new certificates (with new numbers) in the same envelope yesterday with a letter stating the following:
"The Airmen Certification Branch is now issuing all NEW airmen certificates with unique certificate numbers. We will not use your Social Security Number (SSN) for a certificate number.
If you hold a pilot certificate that uses your SSN as your certificate number, and you obtain a flight instructor certificate, we will change your certificate number and issue you a new pilot certificate as well as your flight instructor certificate.
Airmen certificates using the SSN as a certificate number will continue to be issued with that number unless you request that a unique number be assigned.
If you would like to change your certificate number to a unique number, you can print a form for that purplse from our web site at
http://registry.faa.gov/docs/ssn_Change.pdf .
If you have any questions or require further assistance, please contact the Airmen Certification Branch at (405) 954-3261."
The instructor certificate was dated 8 Jan, & the commercial certificate was dated 24 Feb. Those dates (& yesterday) probably indicate the processing times.
Hope this helps.