For a request like this where you are conditionally setting an autonumber on an given record the approach that I have used would not involve having the autonumber on the entity/table itself. Follow this approach:
i. Create a new record in TABLE-NEW which will issue a new autonumber into NEW_NUMBER in the TABLE-NEW
ii. Update your record in TABLE-A with the NEW_NUMBER from the TABLE-NEW record just created.
This workflow (or you could use flow if you want it to be more modern but not as timely) does the conditional assignment from a separate table.
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