They would do it to punish out groups...
They sure are making alot of lists lately.
The TP USA halftime audience
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If you make the barrier to entry high enough that only people with money can get issued new and secure ssn would be one way.
The more devious thing would to just not allow folks on your "terrorist" list to be allowed new numbers. Now you would have a new way to justify filling your concentration camps.
To be clear I don't think currently anything like this is planned, this would just be the most fucked up way fascists could capitalize on their own fuck up. Also probably the only reason they would consider going through the hassle of a ssn restructuring.