> how will the surrogate key protect you? It will not.

Yes it will. Your changes will be confined to only the table(s) where the natural key is present, not spread across every table where there's a foreign key.

Of course you will still have to deal with the reality that the natural key is now not unique, and model reality, but your implementation work in doing so is far simpler.

In more years than I care to count I've regretted someone using natural keys as a primary key for a table many times, and surrogates never.