This existing unique field will have to be a character of fixed length (VARCHAR(12)) because although it's a numeric value there will be leading zeroes.
Plenty of people are contributing their tuppence-worth regarding the choice of surrogate vs natural primary key.
Can I just point out that your existing unique field is EITHER a numeric value OR it has a fixed number of characters - numbers don't have leading zeros.
If what you have is a number, then perhaps consider int8/numeric types and format appropriately when you display the values.
-- Richard Huxton Archonet Ltd
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