Hmm, I'm not 100% sure if I understand your concern, but perhaps my comment will help.
I recently did a marriage based adjustment of status case for a young man from Taiwan. He was here on F-1 and had OPT. He didn't think he had an alien registration number, but when I looked at his OPT card, sure enough the USCIS had assigned one to him.
When I received his I-485 receipt notice, I saw that the USCIS had issued a new alien registration number to him. My guess is that they have certain "fields" of A numbers they assign to OPT cases, and when the person applies for a greencard, they reassign a new number from a different field of numbers.
So, if you worry is that "I never had an alien registration number and now the USCIS is saying they changed my number", I would not worry about it. My guess (and it is only a guess) is that simply means they have assigned a number to you based on your application for OPT.
Further, when I file an I-485 for a person who actually "never" had an alien registration number before, the automated system says, "We've changed your number" which also lends support to the notion that it is normal (language for the USCIS to use) for them to say "we changed your number" even when assigning a number to a person for the very first time.
Best of luck.
Matthew Udall - Attorney
Talking to your DSO would be a good start.
The information contained in this answer is provided for informational purposes only, and should not be construed as legal advice on any subject matter,not should it be viewed as establishing an attorney client relationship of any kind.
CIS has no obligation to expedite 765s bu they do have a regulatory obligation to issue the EAD in 90 days or temporary work authorization. This is why we try to get F-1 students to apply for OPT as early as possible. After the 90 days schedule an INFOPASS appointment at the local district office and if that does not work ask for a Congressional inquiry which is very easy and free.